refactor(stm32mp1): update backup reg for FWU

refactor(stm32mp1): update backup reg for FWU

Change the backup register used to store FWU parameters from 21 to 10.
This is chosen to have a Read/Write secure and Read non-secure register.
The mapping is also changed: only the first 4 bits will be used to store
the FWU index. The 4 next bits will be used to store count info. The
other bits are reserved.

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Signed-off-by: Nicolas Toromanoff <nicolas.toromanoff@foss.st.com>
Change-Id: I9249768287ec5688ba2d8711ce04d429763543d7