iommu/ipmmu-vmsa: Consistent ->of_xlate() handling

The 32-bit ARM code gets updated to make use of ->of_xlate() and the
code is shared between 64-bit and 32-bit ARM. The of_device_is_available()
check gets dropped since it is included in of_iommu_xlate().

Suggested-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Magnus Damm <damm+renesas@opensource.se>
Signed-off-by: Joerg Roedel <jroedel@suse.de>