gpio: mpc8xxx: constify of_device_id

This variable is not modified in the driver and all functions it it
passed to don't change it either. So it can and should be marked const.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>