dmaengine: shdma: separate DMA headers.

Separate SH DMA headers into ones, commonly used by both drivers, and ones,
specific to each of them. This will make the future development of the
dmaengine driver easier.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>