iommu/arm-smmu: Simplify VMID and ASID allocation

We only use ASIDs and VMIDs to identify individual stage-1 and stage-2
context-banks respectively, so rather than allocate these separately
from the context-banks, just calculate them based on the context bank
index.

Note that VMIDs are offset by 1, since VMID 0 is reserved for stage-1.
This doesn't cause us any issues with the numberspaces, since the
maximum number of context banks is half the minimum number of VMIDs.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Joerg Roedel <joro@8bytes.org>