ARM: dts: stm32: 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, LEDs and a custom
board-to-board expansion connector.

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>