ARM: dts: bcm28155-ap: Fix Card Detection GPIO

The board schematic states that the "SD_CARD_DET_N gets pulled to GND
when card is inserted" so the polarity has been updated to active low.

Polarity is now specified with a GPIO define instead of a magic number.

Signed-off-by: Tim Kryger <tim.kryger@linaro.org>
Reviewed-by: Matt Porter <matt.porter@linaro.org>
Signed-off-by: Olof Johansson <olof@lixom.net>