iommu/msm: Use devm_ioremap_resource to simplify code

Use devm_ioremap_resource() to make the code simpler, drop unused variable,
redundant return value check, and error-handing code.

Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>