iommu/arm-smmu-v3: Add explicit feature for nesting

iommu/arm-smmu-v3: Add explicit feature for nesting

commit 1d9777b9f3d55b4b6faf186ba4f1d6fb560c0523 upstream

In certain cases we may want to refuse to allow nested translation even
when both stages are implemented, so let's add an explicit feature for
nesting support which we can control in its own right. For now this
merely serves as documentation, but it means a nice convenient check
will be ready and waiting for the future nesting code.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Reviewed-by: Nicolin Chen <nicolinc@nvidia.com>
Link: https://lore.kernel.org/r/136c3f4a3a84cc14a5a1978ace57dfd3ed67b688.1683731256.git.robin.murphy@arm.com
Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Easwar Hariharan <eahariha@linux.microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>