mmc: add runtime and system power-management support to the MMCIF driver

Adding support for runtime power-management to the MMCIF driver allows
it to save power as long as no card is present. To also allow to turn
off the power domain at that time, we release DMA channels during that
time, since on some sh-mobile systems the DMA controller(s) and the
MMCIF block belong to the same power domain. System-wide power
management has been tested with experimental PM patches on AP4-based
systems.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Chris Ball <cjb@laptop.org>