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>