In order to correctly manage the bring-up of non boot CPUs, the RTCAPB
clock needs to be enabled.
It controls the access to backup registers, where the CPU entrypoint
will be stored.

Change-Id: Ifeeceb4faf64bc9e0778030444f437cc0bb27272
Signed-off-by: Yann Gautier <yann.gautier@st.com>
Signed-off-by: Etienne Carriere <etienne.carriere@st.com>
Signed-off-by: Nicolas Le Bayon <nicolas.le.bayon@st.com>