gpio/gpio-omap.c: add a const qualifier

This prepares *of_device_id.data becoming const. Without this change
the following warning would occur:

	drivers/gpio/gpio-omap.c: In function 'omap_gpio_probe':
	drivers/gpio/gpio-omap.c:1060: warning: assignment discards qualifiers from pointer target type
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>

drivers/gpio/gpio-omap.c

@@ -1058,7 +1058,7 @@ static int __devinit omap_gpio_probe(struct platform_device *pdev)
 	struct device *dev = &pdev->dev;
 	struct device_node *node = dev->of_node;
 	const struct of_device_id *match;
-	struct omap_gpio_platform_data *pdata;
+	const struct omap_gpio_platform_data *pdata;
 	struct resource *res;
 	struct gpio_bank *bank;
 	int ret = 0;