xen/arm: mm: Introduce _PAGE_PRESENT and _PAGE_POPULATE

xen/arm: mm: Introduce _PAGE_PRESENT and _PAGE_POPULATE

At the moment, the flags are not enough to describe what kind of update
will done on the VA range. They need to be used in conjunction with the
enum xenmap_operation.

It would be more convenient to have all the information for the update
in a single place.

Two new flags are added to remove the relience on xenmap_operation:
    - _PAGE_PRESENT: Indicate whether we are adding/removing the mapping
    - _PAGE_POPULATE: Indicate whether we only populate page-tables

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