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>