iommu/io-pgtable-arm: Use for_each_set_bit to simplify the code

We can use for_each_set_bit() to simplify the code slightly in the
ARM io-pgtable self tests.

Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>