ARM: dts: imx6qdl-udoo: Properly describe the SD card detect

GPIO7_IO00 is used as SD card detect.

Properly describe this in the devicetree.

Fixes: 40cdaa54

 ("ARM: dts: imx6q-udoo: Add initial board support")
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>