stm32mp1: split code between common and private parts

Some parts of code could be shared with platform derivatives,
or new platforms.
A new folder plat/st/common is created to put common parts.

stm32mp_common.h is a common API aggregate.

Remove some casts where applicable.
Fix some types where applicable.
Remove also some platform includes that are already in stm32mp1_def.h.

Change-Id: I46d763c8d9e15732d1ee7383207fd58206d7f583
Signed-off-by: Yann Gautier <yann.gautier@st.com>
Signed-off-by: Etienne Carriere <etienne.carriere@st.com>