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/st/common/Makefile b/board/st/common/Makefile
index aa030bac..012bfbb 100644
--- a/board/st/common/Makefile
+++ b/board/st/common/Makefile
@@ -4,6 +4,7 @@
 #
 
 obj-$(CONFIG_CMD_STBOARD) += cmd_stboard.o
+obj-$(CONFIG_PMIC_STPMIC1) += stpmic1.o
 
 ifeq ($(CONFIG_ARCH_STM32MP),y)
 obj-$(CONFIG_SYS_MTDPARTS_RUNTIME) += stm32mp_mtdparts.o