gpio: mpc8xxx: Convert mpc8xxx_gpio_chip.lock to raw_spinlock

mpc8xxx_gpio_chip.lock needs to be a real spinlock in preempt-rt.
Especially the interrupt related functions can not be converted to a
sleeping lock.

Signed-off-by: Alexander Stein <alexander.stein@systec-electronic.com>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>