feat(st): introduce SoC family compilation switch

add STM32MP1X and STM3MP2X compilation switch to replace
#if STM32MP21 || STM32MP23 || STM32MP25 for MP2 SoCs and
#if STM32MP13 || STM32MP15 for MP1 SoCs.

This will avoid to forget to modify all these files when a new SoC is
introduced.

Change-Id: Ib984b22a19e08af5bc1b62fe2032f10240ec9122
Signed-off-by: Maxime Méré <maxime.mere@foss.st.com>
diff --git a/plat/st/stm32mp2/platform.mk b/plat/st/stm32mp2/platform.mk
index 249b796..c20435b 100644
--- a/plat/st/stm32mp2/platform.mk
+++ b/plat/st/stm32mp2/platform.mk
@@ -30,6 +30,8 @@
 # Default Device tree
 DTB_FILE_NAME			?=	stm32mp257f-ev1.dtb
 
+TF_CFLAGS			+=	-DSTM32MP2X
+
 STM32MP21			?=	0
 STM32MP23			?=	0
 STM32MP25			?=	0