iommu/ipmmu-vmsa: Remove redundant of_iommu_init_fn hook

Having of_iommu_init() call ipmmu_init() via ipmmu_vmsa_iommu_of_setup()
does nothing that the subsys_initcall wouldn't do slightly later anyway,
since probe-deferral of masters means it is no longer critical to
register the driver super-early. Clean it up.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>