iommu/arm-smmu-v3: check return value after calling platform_get_resource()

It will cause null-ptr-deref if platform_get_resource() returns NULL,
we need check the return value.

Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Link: https://lore.kernel.org/r/20220425114525.2651143-1-yangyingliang@huawei.com
Signed-off-by: Will Deacon <will@kernel.org>