mmc: slot-gpio: Allow host driver to provide isr for card-detect interrupts

One of the reasons omap_hsmmc doesn't use the slot-gpio library
is that it has some non-standard functionality in the card-detect
interrupt service routine.

To make it possible for omap_hsmmc (and maybe others) to be converted
to use slot-gpio, add 'mmc_gpio_request_cd_isr' which provide an
alternate isr to be register by the slot-gpio code.

Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>