gpio/mpc8xxx: add a const qualifier

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

	drivers/gpio/gpio-mpc8xxx.c: In function 'mpc8xxx_add_controller':
	drivers/gpio/gpio-mpc8xxx.c:360:30: warning: assignment discards 'const' qualifier from pointer target type [enabled by default]

Acked-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>