feat(stm32mp1): enable format-signedness warning

Add the flag -Wformat-signedness to TF_CFLAGS for STM32MP1.

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Change-Id: I6af18778902b0a4dae1c08735d2d070ef3d137ce
diff --git a/plat/st/stm32mp1/platform.mk b/plat/st/stm32mp1/platform.mk
index 8d4320a..ea8ed91 100644
--- a/plat/st/stm32mp1/platform.mk
+++ b/plat/st/stm32mp1/platform.mk
@@ -38,6 +38,7 @@
 endif
 
 TF_CFLAGS		+=	-Wsign-compare
+TF_CFLAGS		+=	-Wformat-signedness
 
 # Not needed for Cortex-A7
 WORKAROUND_CVE_2017_5715:=	0