gpio/gpio-omap: make platformdata used as *of_device_id.data const

Since of_device_id.data is declared as a pointer to const data a few
more consts can be added in this driver.

Acked-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>