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
1 file changed