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>

drivers/gpio/gpio-mxs.c

@@ -49,6 +49,8 @@
 #define GPIO_INT_LEV_MASK	(1 << 0)
 #define GPIO_INT_POL_MASK	(1 << 1)
 
+#define irq_to_gpio(irq)	((irq) - MXS_GPIO_IRQ_START)
+
 struct mxs_gpio_port {
 	void __iomem *base;
 	int id;