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/common/Kconfig b/board/st/common/Kconfig
index 1824087..af01ca4 100644
--- a/board/st/common/Kconfig
+++ b/board/st/common/Kconfig
@@ -1,6 +1,7 @@
 config CMD_STBOARD
 	bool "stboard - command for OTP board information"
-	default y
+	depends on ARCH_STM32MP
+	default y if TARGET_ST_STM32MP15x
 	help
 	  This compile the stboard command to
 	  read and write the board in the OTP.