stm32mp1: Add sign-compare warning

Add -Wsign-compare to TF_CFLAGS to check signedness comparison during
STM32MP1 platform compilation.

Change-Id: I4cada49622f44258d3e0da4560a566de9c7d54b3
Signed-off-by: Yann Gautier <yann.gautier@st.com>
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/tf-a/+/183876


Reviewed-by: Lionel DEBIEVE <lionel.debieve@st.com>
Reviewed-by: CIBUILD <smet-aci-builds@lists.codex.cro.st.com>

plat/st/stm32mp1/platform.mk

@@ -42,6 +42,8 @@ ifeq ($(AARCH32_SP),sp_min)
 TF_CFLAGS		+=	-mfloat-abi=soft
 endif
 
+TF_CFLAGS		+=	-Wsign-compare
+
 # Not needed for Cortex-A7
 WORKAROUND_CVE_2017_5715:=	0