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

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

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
diff --git a/drivers/gpio/gpio-mxc.c b/drivers/gpio/gpio-mxc.c
index 64aff20..b588f8a 100644
--- a/drivers/gpio/gpio-mxc.c
+++ b/drivers/gpio/gpio-mxc.c
@@ -31,6 +31,8 @@
 #include <linux/of_device.h>
 #include <asm-generic/bug.h>
 
+#define irq_to_gpio(irq)	((irq) - MXC_GPIO_IRQ_START)
+
 enum mxc_gpio_hwtype {
 	IMX1_GPIO,	/* runs on i.mx1 */
 	IMX21_GPIO,	/* runs on i.mx21 and i.mx27 */