Some MXS based boards do not implement the card-detect signal. Allow
user to specify alternate card-detect implementation.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Otavio Salvador <otavio@ossystems.com.br>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Cc: Stefano Babic <sbabic@denx.de>