ARM: dts: stm32: Rework DDR DT inclusion

Adjust the DDR configuration dtsi such that they only generate the
DRAM configuration node, the DDR controller node is moved into the
stm32mp157-u-boot.dtsi itself. This permits including multiple DDR
configuration dtsi files in board DT.

Reviewed-by: Patrick Delaunay <patrick.delaunay@st.com>
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>