mmc: sh_mmcif: support generic card-detection

Extend the sh_mmcif driver to support GPIO card detection, provided by the
slot function module. The original .get_cd() platform callback is also
preserved for now.

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