sparc: remove arch specific dma_supported implementations

Usually dma_supported decisions are done by the dma_map_ops instance.
Switch sparc to that model by providing a ->dma_supported instance for
sbus that always returns false, and implementations tailored to the sun4u
and sun4v cases for sparc64, and leave it unimplemented for PCI on
sparc32, which means always supported.

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