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

fdts/stm32mp15-ddr.dtsi

@@ -5,7 +5,7 @@
 
 / {
 	soc {
-		ddr: ddr@5A003000{
+		ddr: ddr@5a003000{
 
 			compatible = "st,stm32mp1-ddr";
 

fdts/stm32mp157c-security.dtsi

@@ -6,7 +6,7 @@
 
 / {
 	soc {
-		stgen: stgen@5C008000 {
+		stgen: stgen@5c008000 {
 			compatible = "st,stm32-stgen";
 			reg = <0x5C008000 0x1000>;
 			status = "okay";