xen/arm: Allow lpae_is_{table, mapping} helpers to work on invalid entry

xen/arm: Allow lpae_is_{table, mapping} helpers to work on invalid entry

Currently, lpae_is_{table, mapping} helpers will always return false on
entries with the valid bit unset. However, it would be useful to have them
operating on any entry. For instance to store information in advance but
still request a fault.

With that change, the p2m is now providing an overlay for *_is_{table,
mapping} that will check the valid bit of the entry.

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