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>

doc/board/index.rst

@@ -42,6 +42,7 @@ Board-specific doc
    renesas/index
    rockchip/index
    samsung/index
+   sielaff/index
    siemens/index
    sifive/index
    sipeed/index

doc/board/sielaff/imx6dl-sielaff.rst

+.. 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.

doc/board/sielaff/index.rst

+.. SPDX-License-Identifier: GPL-2.0+
+
+Sielaff
+=======
+
+.. toctree::
+   :maxdepth: 2
+
+   imx6dl-sielaff