sparc: use DT node full_name in sparc_dma_alloc_resource

The sparc tree already has this change for the pre-refactored code,
but pulling it into the dma-mapping tree like this should ease
the merge conflicts a bit.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: David Miller <davem@davemloft.net>