board_f.c: Ensure gd->new_bootstage alignment

In reserve_bootstage(), in case size is odd, gd->new_bootstage
is not aligned. In bootstage_relocate(), the platform hangs when
getting access to data->record[i].name.
To avoid this issue, make gd->new_bootstage 16 byte aligned.

To ensure that new_bootstage is 16 byte aligned (at least needed for
x86_64 and ARMv8) and new_bootstage starts down to get enough space,
ALIGN_DOWN macro is used.

Fixes: ac9cd480 ("bootstage: Correct relocation algorithm")

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Reviewed-by: Vikas MANOCHA <vikas.manocha@st.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@st.com>
Tested-by: Patrick Delaunay <patrick.delaunay@st.com>
Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>