xen/arm: mm: Add more ASSERT() in {destroy, modify}_xen_mappings()

xen/arm: mm: Add more ASSERT() in {destroy, modify}_xen_mappings()

Both destroy_xen_mappings() and modify_xen_mappings() will take in
parameter a range [start, end[. Both end should be page aligned.

Add extra ASSERT() to ensure start and end are page aligned. Take the
opportunity to rename 'v' to 's' to be consistent with the other helper.

Signed-off-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>