arm: stm32mp: move code for STM32MP15x

Move code and defines only needed for CONFIG_STM32MP15x in stm32mp15x.c
when low level init without TFABOOT is supported.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
diff --git a/arch/arm/mach-stm32mp/Makefile b/arch/arm/mach-stm32mp/Makefile
index 391b47c..d362104 100644
--- a/arch/arm/mach-stm32mp/Makefile
+++ b/arch/arm/mach-stm32mp/Makefile
@@ -8,6 +8,8 @@
 obj-y += syscon.o
 obj-y += bsec.o
 
+obj-$(CONFIG_STM32MP15x) += stm32mp15x.o
+
 ifdef CONFIG_SPL_BUILD
 obj-y += spl.o
 obj-y += tzc400.o