ARM: dts: stm32: Split Avenger96 into DHCOR SoM and Avenger96 board

The Avenger96 is in fact an assembly of DH Electronics DHCOR SoM on top
of an Avenger96 reference board. The DHCOR SoM can be populated with any
STM32MP15xx. Split the DTs to reflect this such that the common SoM and
Avenger96 parts are now in stm32mp15xx-dhcor-*dtsi and a specific example
implementation of STM32MP157A SoM and Avenger96 board is separated into
stm32mp157a-dhcor-*dts* . The stm32mp157a-avenger96.dts is retained for
the sake of backward naming compatibility.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Alexandre Torgue <alexandre.torgue@st.com>
Cc: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
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
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>