xen/arm: mm: Rework xen_pt_update_entry to avoid use xenmap_operation

xen/arm: mm: Rework xen_pt_update_entry to avoid use xenmap_operation

With the newly introduced flags, it is now possible to know how the page
will be updated through the flags.

All the use of xenmap_operation are now replaced with the flags. At the
same time, validity check are now removed as they are gathered in
xen_pt_check_entry().

Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Andrii Anisov <andrii_anisov@epam.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>