ARM: dts: stm32: Split SoC-independent parts of DHCOM SOM and PDK2

The DH Electronics PDK2 can be populated with SoM with any STM32MP15xx
variant. Split the SoC-independent parts of the SoM and PDK2 into the
stm32mp15xx-dhcom-*.dtsi and reduce stm32mp157c-dhcom-*dts* to example
of adding STM32MP157C variant of the SoM into a PDK2 carrier board.

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>