iommu: Clean up of_iommu_init_fn

Now that no more drivers rely on arbitrary early initialisation via an
of_iommu_init_fn hook, let's clean up the redundant remnants. The
IOMMU_OF_DECLARE() macro needs to remain for now, as the probe-deferral
mechanism has no other nice way to detect built-in drivers before they
have registered themselves, such that it can make the right decision.

Reviewed-by: Sricharan R <sricharan@codeaurora.org>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>