iommu/arm-smmu: Fix 16-bit ASID configuration

16-bit ASID should be enabled before initializing TTBR0/1,
otherwise only LSB 8-bit ASID will be considered. Hence
moving configuration of TTBCR register ahead of TTBR0/1
while initializing context bank.

Signed-off-by: Sunil Goutham <sgoutham@cavium.com>
[will: rewrote comment]
Signed-off-by: Will Deacon <will.deacon@arm.com>