mmc: tegra2: Implement card-detect hook.

On Tegra2, card-detection is implemented by passing the card-detection
GPIOs to the MMC driver at initialization time. Instead of implementing
the board_mmc_getcd() function, use the card-detect hook and allow
boards to override it by providing their own board_mmc_getcd()
implementation.

Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>