libcflat: add IS_ALIGNED() macro, and page sizes

These macros will be useful to do page alignment checks.

Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
[drew: also added SZ_64K and changed to shifts]
Signed-off-by: Andrew Jones <drjones@redhat.com>