fix(stm32mp1): rework switch/case for MISRA

Avoid the use of return inside switch/case in stm32mp_is_single_core().
Although this MISRA rulre might not be enforced, we align on what is done
for stm32mp_is_auth_supported().

Change-Id: I00a5ec1b18c55b4254af00c9c5cf5a4dce104175
Signed-off-by: Yann Gautier <yann.gautier@st.com>