ARM: dts: imx6ull-dhcom: Add DHCOM based PicoITX board

Add DT for DH PicoITX unit, which is a bare-bones carrier board for
the DHCOM. The board has ethernet port, USB, CAN, LED and a custom
board-to-board expansion connector. For this board a DHCOM i.MX6ULL
SoM configuration without WiFi/BT is used. The interface is used
for the SD card instead. Make this adjustment by using a separate DT
include file.

Signed-off-by: Christoph Niedermaier <cniedermaier@dh-electronics.com>
Reviewed-by: Marek Vasut <marex@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>