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>