mmc: davinci: Add support for set_power callback

Some devices connected to the MMC bus are power controlled by external
means. For instance, an SDIO device may be powered down/up by an
external gpio line.

In order to avoid toggling power from within the MMC host driver, add a
set_power callback function, which will be called by set_ios upon
powering down/up.

Signed-off-by: Ido Yariv <ido@wizery.com>
CC: Chris Ball <cjb@laptop.org>
Acked-by: Chris Ball <cjb@laptop.org>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>