of: iommu: Silence misleading warning

Printing "IOMMU is currently not supported for PCI" for
every PCI device probed on a DT-based system proves to be
both irritatingly noisy and confusing to users who have
misinterpreted it to mean they can no longer use VFIO device
assignment.

Since configuring DMA masks for PCI devices via
of_dma_configure() has not in fact changed anything with
regard to IOMMUs there really is nothing to warn about here;
shut it up.

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