sh: implement DMA_SLAVE capability in SH dmaengine driver

Tested to work with a SIU ASoC driver on sh7722 (migor).

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>