ARM: dts: stm32: Split AV96 into DHCOR SoM and AV96 board

ARM: dts: stm32: Split AV96 into DHCOR SoM and AV96 board

The AV96 is in fact an assembly of DH Electronics DHCOR SoM on top
of an AV96 reference board. Split the DTs to reflect that and make
sure to DHCOR SoM can be reused on other boards easily.

It is also highly recommended to configure the board for the DHCOM
make stm32mp15_dhcom_basic_defconfig
make DEVICE_TREE=stm32mp15xx-dhcor-avenger96
as that permits reusing the board code for the DH components, like
accessing and reading out the ethernet MAC from EEPROM.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Cc: Patrick Delaunay <patrick.delaunay@st.com>
Cc: Patrice Chotard <patrice.chotard@st.com>
Change-Id: I7db47280d4eb0d668eb4e006355240271154f97f