iommu/arm-smmu-v3: Use DEFINE_RES_MEM() to simplify code

No functional change.

Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Link: https://lore.kernel.org/r/20210122131448.1167-1-thunder.leizhen@huawei.com


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