gpio: mpc8xxx: handle differences between incarnations at a single place

The gpio controllers that are handled by the mpc8xxx driver differ
slightly. Up to now some differences were handled by use of
of_device_is_compatible, others by use of struct of_device_id's data.

To make this consistent and easily extendable handle the differences at
a single place.

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