mmc: sdio: enable runtime PM for SDIO cards

Enable runtime PM for new SDIO cards.

As soon as the card will be added to the device tree, runtime PM core
will release its power, since it doesn't have any users yet.

Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>
Tested-by: Luciano Coelho <luciano.coelho@nokia.com>
Signed-off-by: Chris Ball <cjb@laptop.org>