arm/mxs: move MXS_GPIO_NR into mach/mxs.h

The patch moves MXS_GPIO_NR into mach/mxs.h, so that we only use
standard gpiolib helpers in mach/gpio.h.

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