ARM: nomadik: move MMC/SD card detect GPIO to board DTS

This pushes the setting of the card detect GPIO pin down into
the top-level file for the board, since it is not a property of
the ASIC (which this DTSI is about) but a property of the board
design.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>