board: stm32mp1: move board_get_mtdparts in st common directory

Move the stm32mp1 common code board_get_mtdparts() 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/dhelectronics/dh_stm32mp1/Makefile b/board/dhelectronics/dh_stm32mp1/Makefile
index b42c4e4..c77a1e3 100644
--- a/board/dhelectronics/dh_stm32mp1/Makefile
+++ b/board/dhelectronics/dh_stm32mp1/Makefile
@@ -8,3 +8,4 @@
 endif
 
 obj-y += ../../st/stm32mp1/board.o board.o
+obj-$(CONFIG_SYS_MTDPARTS_RUNTIME) += ../../st/common/stm32mp_mtdparts.o