ARM: dts: stm32mp1: correct ddr node

This patch fix the warning:
dt.dts: Warning (simple_bus_reg): Node /soc/ddr@5A003000
simple-bus unit address format error, expected "5a003000"

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@st.com>