fix(stm32mp1): fix enum prints

With gcc-11, the -Wformat-signedness warning complains about enum values
that should be printed as unsigned values. But the current version of
compiler used in CI states that this parameter is signed. Just cast the
value then.

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Change-Id: Ic0655e5ba9c44fe6abcd9958d7a9972f5de3b7ef