iommu/arm-smmu: Rename public #defines under ARM_SMMU_ namespace

Now that we have arm-smmu.h defining various SMMU constants, ensure that
they are namespaced with the ARM_SMMU_ prefix in order to avoid conflicts
with the CPU, such as the one we're currently bodging around with the
TCR.

Cc: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>