iommu/mediatek: Fix NULL pointer dereference when printing dev_name

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

iommu/mediatek: Fix NULL pointer dereference when printing dev_name

When larbdev is NULL (in the case I hit, the node is incorrectly set
iommus = <&iommu NUM>), it will cause device_link_add() fail and
kernel crashes when we try to print dev_name(larbdev).

Let's fail the probe if a larbdev is NULL to avoid invalid inputs from
dts.

It should work for normal correct setting and avoid the crash caused
by my incorrect setting.

Error log:
[ 18.189042][ T301] Unable to handle kernel NULL pointer dereference at virtual address 0000000000000050
...
[ 18.344519][ T301] pstate: a0400005 (NzCv daif +PAN -UAO)
[ 18.345213][ T301] pc : mtk_iommu_probe_device+0xf8/0x118 [mtk_iommu]
[ 18.346050][ T301] lr : mtk_iommu_probe_device+0xd0/0x118 [mtk_iommu]
[ 18.346884][ T301] sp : ffffffc00a5635e0
[ 18.347392][ T301] x29: ffffffc00a5635e0 x28: ffffffd44a46c1d8
[ 18.348156][ T301] x27: ffffff80c39a8000 x26: ffffffd44a80cc38
[ 18.348917][ T301] x25: 0000000000000000 x24: ffffffd44a80cc38
[ 18.349677][ T301] x23: ffffffd44e4da4c6 x22: ffffffd44a80cc38
[ 18.350438][ T301] x21: ffffff80cecd1880 x20: 0000000000000000
[ 18.351198][ T301] x19: ffffff80c439f010 x18: ffffffc00a50d0c0
[ 18.351959][ T301] x17: ffffffffffffffff x16: 0000000000000004
[ 18.352719][ T301] x15: 0000000000000004 x14: ffffffd44eb5d420
[ 18.353480][ T301] x13: 0000000000000ad2 x12: 0000000000000003
[ 18.354241][ T3…