iommu/arm-smmu: ensure CBA2R is initialised before CBAR on SMMUv2

The VMID16 (8.1) extension to SMMUv2 added a 16-bit VMID16 field to the
CBA2R registers. Unfortunately, if software writes this field as zero
after setting an 8-bit VMID in a stage-2 CBAR, then the VMID may also be
overwritten with zero on some early implementations (the architecture
was later updated to fix this issue).

This patch ensures that we initialise CBA2R before CBAR, therefore
ensuring that the VMID is set correctly.

Tested-by: Manish Jaggi <mjaggi@caviumnetworks.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>