ARM: davinci: da850-evm: use gpio descriptor for mmc pins

Currently the mmc driver is polling the gpio to know if the
card was removed.

By using a gpio descriptor instead of the platform callbacks,
the driver will be able to register the gpio using the mmc core
APIs designed for this purpose.

This has the advantage that an irq will be registered, and
polling is no longer needed. Also, a dependency on platform
callbacks is removed for this board.

Signed-off-by: Axel Haslam <ahaslam@baylibre.com>
[nsekhar@ti.com: minor commit message edit]
Signed-off-by: Sekhar Nori <nsekhar@ti.com>