sparc64/pci_sun4v: allow large DMA masks

We've been moving to a model where the device just sets the DMA mask
supported by it, instead of having to fallback to something it thinks
the platform might support.  Sparc64 is the remaining holdout forcing
drivers to supply a matching mask.  Change dma_4v_supported to just
check if the supplied dma mask is large enough. and adjust the mapping
code to check ATU presence in addition to the DMA mask to decide on
the mapping method.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: David S. Miller <davem@davemloft.net>