xen/mmu: set MMU_NORMAL_PT_UPDATE in remap_area_mfn_pte_fn

No functional change because MMU_NORMAL_PT_UPDATE is in fact 0. Set it
to make the code consistent with similar code in mmu_pv.c

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>