mmc: add a card hotplug handler context

SD/MMC controllers provide different card insertion and removal detection
methods. On some of them the controller itself issues an interrupt, on
others polling is used, on yet others auxiliary means are used for this
purpose, e.g., a GPIO IRQ. Further, on some systems one of those methods
can be chosen at driver probing time and configured in software. E.g., on
some systems the SD/MMC controller card hot-plug detection pin can be
configured either as a respective controller functions, or an IRQ-capable
GPIO. To support such flexible configurations a card hot-plug context
is added by this patch.

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