iommu/ipmmu-vmsa: Refactor micro-TLB lookup

Cache the micro-TLB number in archdata allocated in the .add_device
handler instead of looking it up when the deviced is attached and
detached. This simplifies the .attach_dev and .detach_dev operations and
prepares for DT support.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>