ARM: dts: imx6ull-dhcom: Add DH electronics DHCOM i.MX6ULL SoM and PDK2 board

Add support for DH electronics DHCOM SoM and PDK2 rev. 400 carrier
board. This is an SoM with i.MX6ULL and an evaluation kit. The
baseboard provides Ethernet, UART, USB, CAN and optional display.

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