sparc32: page align size in arch_dma_alloc

Commit 53b7670e

 ("sparc: factor the dma coherent mapping into
helper") lost the page align for the calls to dma_make_coherent and
srmmu_unmapiorange. The latter cannot handle a non page aligned len
argument.

Signed-off-by: Andreas Larsson <andreas@gaisler.com>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>