stm32mp1: check stronger the secondary CPU entry point

When using SP_min as monitor, only sp_min_warm_entrypoint() is a valid
secure entry point.

Change-Id: I440cec798e901b11a34dd482c33b2e378a8328ab
Signed-off-by: Etienne Carriere <etienne.carriere@st.com>
Signed-off-by: Nicolas Toromanoff <nicolas.toromanoff@st.com>