mmc: sdhci-of-at91: fix wake-up issue when using runtime pm

It is impossible to wake-up on card detect event because when sdhci
controller is runtime suspended, it is assumed that all clocks are
disabled so we can't get irqs.
If the device is removable and there is no gpio to manage the card
detection then polling is used. It doesn't mean card detection is broken.
It is curently we only way to wake-up on card event if using runtime pm.

Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>