board/adp-ag102: add configuration of adp-ag102

board:
Add config file of board adp-ag102
Add adp-ag102 into boards.cfg
Add adp-ag102 into MAINTAINERS

doc:
add README of ag102

Signed-off-by: Macpaul Lin <macpaul@andestech.com>
diff --git a/doc/README.ag102 b/doc/README.ag102
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+Andes Technology SoC AG102
+==========================
+
+AG102 is the second SoC produced by Andes Technology using N1213 CPU core
+with FPU and DDR contoller support.
+AG102 has integrated both AHB and APB bus and many periphals for application
+and product development.
+
+ADP-AG102
+=========
+
+ADP-AG102 is the SoC with AG102 hardcore CPU.
+
+Configurations
+==============
+
+CONFIG_MEM_REMAP:
+	Doing memory remap is essential for preparing some non-OS or RTOS
+	applications.
+
+CONFIG_SKIP_LOWLEVEL_INIT:
+	If you want to boot this system from SPI ROM and bypass e-bios (the
+	other boot loader on ROM). You should undefine CONFIG_SKIP_LOWLEVEL_INIT
+	in "include/configs/adp-ag102.h".
+
+Build and boot steps
+====================
+
+build:
+1. Prepare the toolchains and make sure the $PATH to toolchains is correct.
+2. Use `make adp-ag102` in u-boot root to build the image.
+
+Burn u-boot to SPI ROM:
+====================
+
+This section will be added later.