board: stm32mp1: move set_dfu_alt_info in st common directory

Move the stm32mp1 common code set_dfu_alt_info() in common directory,
this patch reduce the maintenance effort on this generic part (not board
dependent).

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@st.com>
diff --git a/board/st/common/Makefile b/board/st/common/Makefile
index 4bb8b49..aa030bac 100644
--- a/board/st/common/Makefile
+++ b/board/st/common/Makefile
@@ -7,4 +7,5 @@
 
 ifeq ($(CONFIG_ARCH_STM32MP),y)
 obj-$(CONFIG_SYS_MTDPARTS_RUNTIME) += stm32mp_mtdparts.o
+obj-$(CONFIG_SET_DFU_ALT_INFO) += stm32mp_dfu.o
 endif