iommu/arm-smmu: ioremap smmu mmio region before implementation init

ioremap smmu mmio region before calling into implementation init.
This is necessary to allow mapped address available during vendor
specific implementation init.

Signed-off-by: Krishna Reddy <vdumpa@nvidia.com>
Reviewed-by: Jon Hunter <jonathanh@nvidia.com>
Reviewed-by: Nicolin Chen <nicoleotsuka@gmail.com>
Reviewed-by: Pritesh Raithatha <praithatha@nvidia.com>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Reviewed-by: Thierry Reding <thierry.reding@gmail.com>
Link: https://lore.kernel.org/r/20200718193457.30046-3-vdumpa@nvidia.com
Signed-off-by: Will Deacon <will@kernel.org>