iommu/arm-smmu: Split out register defines

I want to re-use some of these for qcom_iommu, which has (roughly) the
same context-bank registers.

Signed-off-by: Rob Clark <robdclark@gmail.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>