dmaengine: shdma: add runtime- and system-level power management

This patch extends and fixes runtime power management in the shdma
driver to support powering down the DMA controller and adds support
for system-level suspend and resume.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>