iommu/ipmmu-vmsa: Calculate context registers' offset instead of a macro

iommu/ipmmu-vmsa: Calculate context registers' offset instead of a macro

This is a non-verbatim port of corresponding Linux upsteam commit:
3dc28d9f59eaae41461542b27afe70339347ebb3

Original commit message:
 commit 3dc28d9f59eaae41461542b27afe70339347ebb3
 Author: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
 Date:   Wed Nov 6 11:35:48 2019 +0900

  iommu/ipmmu-vmsa: Calculate context registers' offset instead of a macro

  Since we will have changed memory mapping of the IPMMU in the future,
  this patch uses ipmmu_features values instead of a macro to
  calculate context registers offset. No behavior change.

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

This is a prereq work needed to add support for S4 series easily
in the future.

Almost the same change as original commit makes, but without updating
struct ipmmu_features_default which Xen driver doesn't have (there is
no support of old Arm32 based Gen2 SoCs).

No change in behavior.

Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
Reviewed-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Acked-by: Julien Grall <jgrall@amazon.com>