ARM: dts: stm32: Add DH Electronics DHCOM STM32MP1 SoM and PDK2 board

Add support for DH Electronics DHCOM SoM and PDK2 rev. 400 carrier
board. This is an SoM with STM32MP157C and an evaluation kit. The
baseboard provides Ethernet, UART, USB, CAN and optional display.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Alexandre Torgue <alexandre.torgue@st.com>
Cc: Maxime Coquelin <mcoquelin.stm32@gmail.com>
Cc: Patrice Chotard <patrice.chotard@st.com>
Cc: Patrick Delaunay <patrick.delaunay@st.com>
Cc: linux-stm32@st-md-mailman.stormreply.com
To: linux-arm-kernel@lists.infradead.org
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>