iommu/ipmmu-vmsa: Add utlb_offset_base

Since we will have changed memory mapping of the IPMMU in the future,
this patch adds a utlb_offset_base into struct ipmmu_features
for IMUCTR and IMUASID registers. No behavior change.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Joerg Roedel <jroedel@suse.de>