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/board/st/stm32mp1/Kconfig b/board/st/stm32mp1/Kconfig
index 89fc562..c5ab755 100644
--- a/board/st/stm32mp1/Kconfig
+++ b/board/st/stm32mp1/Kconfig
@@ -1,4 +1,4 @@
-if TARGET_STM32MP1
+if TARGET_ST_STM32MP15x
 
 config SYS_BOARD
 	default "stm32mp1"
@@ -9,10 +9,5 @@
 config SYS_CONFIG_NAME
 	default "stm32mp1"
 
-config TARGET_STM32MP157C_DK2
-	bool "support of STMicroelectronics STM32MP157C-DK2 Discovery Board"
-	default y
-
 source "board/st/common/Kconfig"
-
 endif