iommu/mediatek: Move hw_init into attach_device

In attach device, it will update the pagetable base address register.
Move the hw_init function also here. Then it only need call
pm_runtime_get/put one time here if m4u has power domain.

Signed-off-by: Yong Wu <yong.wu@mediatek.com>
Reviewed-by: Tomasz Figa <tfiga@chromium.org>
Link: https://lore.kernel.org/r/20210111111914.22211-17-yong.wu@mediatek.com
Signed-off-by: Will Deacon <will@kernel.org>