gpio/mxs: move irq_to_gpio() into gpio-mxs driver

As irq_to_gpio() is only being used by gpio-mxs driver, it should be
moved from mach/gpio.h into gpio-mxs.c.

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