board: st: create common file stpmic1.c

Move function board_ddr_power_init() in a new file stpmic1 in
board/st/common to avoid duplicated code in each board using
stpmic1

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 e8f218d..5758d98 100644
--- a/board/dhelectronics/dh_stm32mp1/Makefile
+++ b/board/dhelectronics/dh_stm32mp1/Makefile
@@ -7,7 +7,7 @@
 obj-y += ../../st/stm32mp1/spl.o
 endif
 
-obj-y += ../../st/stm32mp1/board.o board.o
+obj-y += ../../st/common/stpmic1.o board.o
 
 obj-$(CONFIG_SYS_MTDPARTS_RUNTIME) += ../../st/common/stm32mp_mtdparts.o
 obj-$(CONFIG_SET_DFU_ALT_INFO) += ../../st/common/stm32mp_dfu.o