Assign the generic runtime PM handlers for SDIO.

These handlers invoke the relevant SDIO function drivers'
handlers, if exist, otherwise they just return success
(so SDIO drivers don't have to define any runtime PM handlers
unless they need to).

Runtime PM is still disabled by default, so this patch alone
has no immediate effect.
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>