mmc: add a card-event host operation

Some hosts need to perform additional actions upon card insertion or
ejection. Add a host operation to be called from card detection handlers.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Reviewed-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Chris Ball <cjb@laptop.org>