dma: sh_dma: not all SH DMAC implementations support MEMCPY

Add a flag to allow platforms to specify, whether a DMAC instance supports
the MEMCPY operation. To avoid regressions, preserve the current default.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Acked-by: Paul Mundt <lethal@linux-sh.org>
Signed-off-by: Vinod Koul <vinod.koul@linux.intel.com>