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/arch/arm/dts/Makefile b/arch/arm/dts/Makefile
index 04a8ccc..8c11173 100644
--- a/arch/arm/dts/Makefile
+++ b/arch/arm/dts/Makefile
@@ -862,7 +862,7 @@
 
 dtb-$(CONFIG_ARCH_STI) += stih410-b2260.dtb
 
-dtb-$(CONFIG_TARGET_STM32MP1) += \
+dtb-$(CONFIG_STM32MP15x) += \
 	stm32mp157a-dk1.dtb \
 	stm32mp157a-avenger96.dtb \
 	stm32mp157c-dk2.dtb \