gpio: mpc8xxx: add support for MPC5125

The gpio controller on MPC5125 is identical to the MPC5121 register
wise, the only difference is that the lines 0..3 are input only instead
of 28..31 on MPC5121.

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