mmc: core: Add bus_ops for runtime pm callbacks

SDIO is the only protocol that uses runtime pm for the card device
right now. To provide the option for sd and mmc to use runtime pm as
well the bus_ops callback are extended with two new functions. One for
runtime_suspend and one for runtime_resume.

This patch will also implement the callbacks for SDIO to make sure
existing functionality is maintained. It also prepares to move
away from using the mmc_power_restore_host API, since it is not
needed when using runtime PM.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Chris Ball <cjb@laptop.org>