iommu/dma: Remove bogus dma_supported() implementation

Back when this was first written, dma_supported() was somewhat of a
murky mess, with subtly different interpretations being relied upon in
various places. The "does device X support DMA to address range Y?"
uses assuming Y to be physical addresses, which motivated the current
iommu_dma_supported() implementation and are alluded to in the comment
therein, have since been cleaned up, leaving only the far less ambiguous
"can device X drive address bits Y" usage internal to DMA API mask
setting. As such, there is no reason to keep a slightly misleading
callback which does nothing but duplicate the current default behaviour;
we already constrain IOVA allocations to the iommu_domain aperture where
necessary, so let's leave DMA mask business to architecture-specific
code where it belongs.

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