stm32mp: add soc.c file

Add a new file soc.c for common functions between stm32mp1 and stm32mp2
family and move print_cpuinfo() in this new file.

Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Igor Opaniuk <igor.opaniuk@foundries.io>