iommu/arm-smmu: Add support for 16 bit VMID

This patch adds support for 16-bit VMIDs on implementations of SMMUv2
that support it.

Signed-off-by: Tirumalesh Chalamarla <tchalamarla@caviumnetworks.com>
[will: commit messsage and comments]
Signed-off-by: Will Deacon <will.deacon@arm.com>