fdts: Fix DTC warnings for STM32MP1 platform

fdts: Fix DTC warnings for STM32MP1 platform

DTC issues below warnings for STM32MP1 platform for using upper case
in unit address:

fdts/stm32mp15-ddr.dtsi:8.20-151.5: Warning (simple_bus_reg): /soc/ddr@5A003000: simple-bus unit address format error, expected "5a003000"
fdts/stm32mp157c-security.dtsi:9.25-13.5: Warning (simple_bus_reg): /soc/stgen@5C008000: simple-bus unit address format error, expected "5c008000"

Fix this by using the lower case unit address for concerned nodes.

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Change-Id: Id3d19ac3b47ec6bcea2bd3382225e2e923dc4a70