mmc: sdhi: Allow named IRQs to use specific handlers

Allow named IRQs to use corresponding specific handlers. If named IRQs are
used, at least an "sdcard" IRQ has to be specified by the platform. If
names are not used, an arbitrary number of IRQs can be provided by the
platform, in which case the generic ISR will be used for each of them.

Cc: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Acked-by: Magnus Damm <magnus.damm@gmail.com>
Signed-off-by: Simon Horman <horms@verge.net.au>
[g.liakhovetski@gmx.de: style and typo corrections, platform data check]
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Chris Ball <cjb@laptop.org>