xen: arm: Support <32MB frametables

xen: arm: Support <32MB frametables

setup_frametable_mappings() rounds frametable_size up to a multiple
of 32MB. This is wasteful on systems with less than 4GB of RAM,
although it does allow the "contig" bit to be set in the PTEs.

Where the frametable is less than 32MB in size, instead round up
to a multiple of 2MB, not setting the "contig" bit in the PTEs.

Signed-off-by: Chris Brand <chris.brand@broadcom.com>
Reviewed-by: Julien Grall <julien.grall@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>