davinci: da830/omap-l137 evm: add support for GPIO based MMC/SD card detection

The DA830/OMAP-L137 EVM has GPIO based card detection logic, but the current
code does not use it.

Add support for GPIO based card detection to avoid reading the card to see
if a card is present or not.

Signed-off-by: Vipin Bhandari <vipin.bhandari@ti.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>