iommu/mediatek: Fix NULL pointer deference in mtk_iommu_device_group

In the Linux kernel, the following vulnerability has been resolved:

iommu/mediatek: Fix NULL pointer deference in mtk_iommu_device_group

Currently, mtk_iommu calls during probe iommu_device_register before

the hw_list from driver data is initialized. Since iommu probing issue

fix, it leads to NULL pointer dereference in mtk_iommu_device_group when

hw_list is accessed with list_first_entry (not null safe).

So, change the call order to ensure iommu_device_register is called

after the driver data are initialized.