iommu/arm-smmu: fix architecture version detection

The SMMU driver was relying on a quirk of MMU-500 r2px to identify
the correct architecture version. Since this does not apply to other
implementations, make the architecture version for each supported
implementation explicit.

While we're at it, remove the unnecessary #ifdef since the dependencies
for CONFIG_ARM_SMMU already imply CONFIG_OF.

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