arm/imx: fix power button on imx51 babbage board

Since commit 6571534b (plat-mxc: iomux-v3.h: implicitly enable
pull-up/down when that's desired) was in, the power button on imx51
babbage board stopped working because it's pulled up by mistake.
The patch removes the pull-up setting from the pad configuration for
that gpio to make the power button back to work.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>