mmc: davinci: request gpios using gpio descriptors

Request card detect and write protect gpios using the provided API
by mmc core.

If a gpio is provided for card detect, we don't need to poll.
So only use polling when a gpio is not provided.

Once all pdata users register the gpios using gpio descriptors,
we could remove the platform callbacks.

Signed-off-by: Axel Haslam <ahaslam@baylibre.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Tested-by: David Lechner <david@lechnology.com>