sh: prepare the DMA driver for slave functionality

Slave DMA functionality uses scatter-gather arrays for data transfers,
whereas memcpy just uses a single data buffer. This patch converts the
current memcpy implementation in shdma.c to use scatter-gather, making it
just a special case with one SG-element. This allows us to isolate
descriptor list manipulations and locking into one function, thus reducing
error chances.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Acked-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>