stm32mp1: split board and SOC support for STM32MP15x family

Split the board and SOC support for STM32MP15x family and
prepare the introduction of new boards with STM32MP15x.

This path define the 2 configurations:
- STM32MP15x: STM32MP15x soc support (new)
- TARGET_ST_STM32MP15x: STMicroelectronics board support (choice)

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