iommu/arm-smmu: ThunderX mis-extends 64bit registers

The SMMU architecture defines two different behaviors when 64-bit
registers are written with 32-bit writes.  The first behavior causes
zero extension into the upper 32-bits.  The second behavior splits a
64-bit register into "normal" 32-bit register pairs.

On some buggy implementations, registers incorrectly zero extended
when they should instead behave as normal 32-bit register pairs.

Signed-off-by: Tirumalesh Chalamarla <tchalamarla@caviumnetworks.com>
[will: removed redundant macro parameters]
Signed-off-by: Will Deacon <will.deacon@arm.com>