iommu/ipmmu-vmsa: Add helper functions for MMU "context" registers

Since we will have changed memory mapping of the IPMMU in the future,
This patch adds helper functions ipmmu_ctx_{reg,read,write}()
for MMU "context" registers. 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>