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>
diff --git a/configs/stm32mp15_optee_defconfig b/configs/stm32mp15_optee_defconfig
index 6a11202..b45462b 100644
--- a/configs/stm32mp15_optee_defconfig
+++ b/configs/stm32mp15_optee_defconfig
@@ -3,7 +3,7 @@
 CONFIG_SYS_MALLOC_F_LEN=0x3000
 CONFIG_ENV_SECT_SIZE=0x40000
 CONFIG_ENV_OFFSET=0x280000
-CONFIG_TARGET_STM32MP1=y
+CONFIG_TARGET_ST_STM32MP15x=y
 CONFIG_STM32MP1_OPTEE=y
 CONFIG_DISTRO_DEFAULTS=y
 CONFIG_FIT=y