doc: board: Add minimal documentation for Sielaff i.MX6 Solo board

Describe how to build and boot for the Sielaff i.MX6 Solo board.

Signed-off-by: Frieder Schrempf <frieder.schrempf@kontron.de>
diff --git a/doc/board/sielaff/imx6dl-sielaff.rst b/doc/board/sielaff/imx6dl-sielaff.rst
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+.. SPDX-License-Identifier: GPL-2.0+
+
+Sielaff i.MX6 Solo Board
+========================
+
+The Sielaff i.MX6 Solo board is a control and Human Machine Interface (HMI)
+board for vending machines.
+
+Quick Start
+-----------
+
+Build U-Boot
+^^^^^^^^^^^^
+
+.. code-block:: bash
+
+   make imx6dl_sielaff_defconfig
+   make CROSS_COMPILE=arm-linux-gnueabi-
+
+Copy the flash.bin file to an SD card at an offset of 1 KiB:
+
+.. code-block:: bash
+
+   dd if=flash.bin of=/dev/sd[x] bs=1K seek=1
+
+Boot
+^^^^
+
+Put the SD card in the slot on the board and apply power.