sh: Use __GFP_ZERO for dma_generic_alloc_coherent().

This follows the x86 change off of memset() and on to an unconditional
__GFP_ZERO for wrapping in to optimized page clearing by way of
clear_highpage().

Signed-off-by: Andrew Murray <amurray@mpc-data.co.uk>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>