iommu/arm-smmu: Use new devm_krealloc()

The implementation-specific subclassing of struct arm_smmu_device really
wanted an appropriate version of realloc(). Now that one exists, take
full advantage of it to clarify what's actually being done here.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/355e8d70c7f47d462d85b386aa09f2b5c655f023.1603713428.git.robin.murphy@arm.com


Signed-off-by: Will Deacon <will@kernel.org>