dts: imx51: fix sd card gpio polarity specified in device tree

cd-gpios polarity should be changed to GPIO_ACTIVE_LOW and wp-gpios
should be changed to GPIO_ACTIVE_HIGH.
Otherwise, the SD may not work properly due to wrong polarity inversion
specified in DT after switch to common parsing function mmc_of_parse().

Signed-off-by: Dong Aisheng <aisheng.dong@freescale.com>
Acked-by: Shawn Guo <shawnguo@kernel.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>