ARM: dts: stm32: Synchronize DDR setttings on DH SoMs

Add custom DDR DRAM settings for the DHCOR and DHCOM SoMs and put them
into use by the board file instead of the default ones. These new DRAM
settings are a better fit for the SoMs.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Patrick Delaunay <patrick.delaunay@st.com>
Cc: Patrice Chotard <patrice.chotard@st.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@st.com>