The micro SD card slot uses GPIO3_13 as card detect pin, so describe
it in the devicetree.

This was noticed when converting imx53-qsb board to driver model
in U-Boot as the micro SD card was not getting detected.
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>