mmc: sh_mmcif: process requests asynchronously

This patch converts the sh_mmcif MMC host driver to process requests
asynchronously instead of waiting in its .request() method for completion.
This is achieved by using threaded IRQs.

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