arm: stm32mp: add choice for STM32MP SOC family

Add mandatory choice for SOC support in ARCH_STM32MP.

This patch is a preliminary step for new SOC introduction
in STM32MP family.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
4 files changed