ARM: dts: stm32: Add DH Electronics DHCOM SoM and PDK2 board

Add support for DH Electronics DHCOM SoM and PDK2 rev. 400 carrier
board. This is an SoM with STM32MP15xx and an evaluation kit. The
baseboard provides Ethernet, UART, USB, CAN and optional display.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Patrick Delaunay <patrick.delaunay@st.com>
Cc: Patrice Chotard <patrice.chotard@st.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@st.com>