arm: stm32mp: add sub config Kconfig.15x

Add sub Kconfig for each SOC in the STM32 CPU family.

It is a preliminary step to introduce a new SOC in the STM32MP family.

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