mx31moboard: invert sdhc ro signal sense

Small confusion with our hardware engineer, the WP signal (RO) is
active low on our boards, the signal has to inverted.

This is a pretty straightforward patch, it could even go to -rc,
but if not, then push it for 2.6.32.

Signed-off-by: Valentin Longchamp <valentin.longchamp@epfl.ch>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>