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>