MCKPROT hardening in RCC mandates that both bits RCC[TZEN] and
RCC[MCKPROT] are enabled. This change fixes stm32mp1_rcc_is_mckprot()
to check both bits, not RCC[MCKPROT] only.

This change also updates stm32mp1_rcc_is_secure() for consistency.

Change-Id: If1f07babdcb5677906ddbf974d9dc17255d4e174
Signed-off-by: Etienne Carriere <etienne.carriere@st.com>
Signed-off-by: Yann Gautier <yann.gautier@st.com>