doc: board: xilinx: Add documentation for ZynqMP R5

Add missing documentation for Xilinx ZynqMP R5 configuration with steps how
to run it.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
diff --git a/doc/board/xilinx/zynqmp-r5.rst b/doc/board/xilinx/zynqmp-r5.rst
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+.. SPDX-License-Identifier: GPL-2.0
+..  (C) Copyright 2020 Xilinx, Inc.
+
+ZYNQMP-R5
+=========
+
+About this
+----------
+
+This document describes the information about Xilinx Zynq UltraScale+ MPSOC
+U-Boot Cortex R5 support.
+
+ZynqMP R5 boards
+----------------
+
+* zynqmp-r5 - U-Boot running on RPU Cortex-R5
+
+Building
+--------
+
+configure and build armv7 toolchain::
+
+   $ make xilinx_zynqmp_r5_defconfig
+   $ make
+
+Notes
+^^^^^
+
+Output fragment is u-boot.
+
+Loading
+-------
+
+ZynqMP R5 U-Boot was created for supporting loading OS on RPU. There are two
+ways how to start U-Boot on R5.
+
+Bootgen
+^^^^^^^
+
+The first way is to use Xilinx FSBL (First stage
+bootloader) to load u-boot and start it. The following bif can be used for boot
+image generation via Xilinx bootgen utility::
+
+
+  the_ROM_image:
+  {
+  	[bootloader,destination_cpu=r5-0] fsbl_rpu.elf
+  	[destination_cpu=r5-0]u-boot.elf
+  }
+
+Bootgen command for building boot.bin::
+
+  bootgen -image <bif>.bif -r -w -o i boot.bin
+
+
+U-Boot cpu command
+^^^^^^^^^^^^^^^^^^
+
+The second way to load U-Boot to Cortex R5 is from U-Boot running on A53 as is
+visible from the following log::
+
+  U-Boot SPL 2020.10-rc4-00090-g801b3d5c5757 (Sep 15 2020 - 14:07:24 +0200)
+  PMUFW:	v1.1
+  Loading new PMUFW cfg obj (2024 bytes)
+  EL Level:	EL3
+  Multiboot:	0
+  Trying to boot from MMC2
+  spl: could not initialize mmc. error: -19
+  Trying to boot from MMC1
+  spl_load_image_fat_os: error reading image u-boot.bin, err - -2
+  NOTICE:  ATF running on XCZU7EG/EV/silicon v4/RTL5.1 at 0xfffea000
+  NOTICE:  BL31: v2.2(release):v2.2-614-ged9dc512fb9c
+  NOTICE:  BL31: Built : 09:32:09, Mar 13 2020
+
+
+  U-Boot 2020.10-rc4-00090-g801b3d5c5757 (Sep 15 2020 - 14:07:24 +0200)
+
+  Model: ZynqMP ZCU104 RevC
+  Board: Xilinx ZynqMP
+  DRAM:  2 GiB
+  PMUFW:	v1.1
+  EL Level:	EL2
+  Chip ID:	zu7e
+  WDT:   Started with servicing (60s timeout)
+  NAND:  0 MiB
+  MMC:   mmc@ff170000: 0
+  Loading Environment from FAT... *** Warning - bad CRC, using default environment
+
+  In:    serial
+  Out:   serial
+  Err:   serial
+  Bootmode: LVL_SHFT_SD_MODE1
+  Reset reason:	SOFT
+  Net:
+  ZYNQ GEM: ff0e0000, mdio bus ff0e0000, phyaddr 12, interface rgmii-id
+  eth0: ethernet@ff0e0000
+  Hit any key to stop autoboot:  0
+  ZynqMP> setenv autoload no
+  ZynqMP> dhcp
+  BOOTP broadcast 1
+  DHCP client bound to address 192.168.0.167 (8 ms)
+  ZynqMP> tftpboot 20000000 192.168.0.105:u-boot-r5-2.elf
+  Using ethernet@ff0e0000 device
+  TFTP from server 192.168.0.105; our IP address is 192.168.0.167
+  Filename 'u-boot-r5-2.elf'.
+  Load address: 0x20000000
+  Loading: #################################################################
+  	 #################################################################
+  	 #################################################################
+  	 #################################################################
+  	 #################################################################
+  	 #################################################################
+  	 ################
+  	 376 KiB/s
+  done
+  Bytes transferred = 2075464 (1fab48 hex)
+  ZynqMP> setenv autostart no
+  ZynqMP> bootelf -p 20000000
+  ZynqMP> cpu 4 release 10000000 lockstep
+  Using TCM jump trampoline for address 0x10000000
+  R5 lockstep mode
+  ZynqMP>
+
+Then on second uart you can see U-Boot up and running on R5::
+
+  U-Boot 2020.10-rc4-00071-g7045622cc9ba (Sep 16 2020 - 13:38:53 +0200)
+
+  Model: Xilinx ZynqMP R5
+  DRAM:  512 MiB
+  MMC:
+  In:    serial@ff010000
+  Out:   serial@ff010000
+  Err:   serial@ff010000
+  Net:   No ethernet found.
+  ZynqMP r5>
+
+Please make sure MIO pins for uart are properly configured to see output.