gpio: mpc8xxx: Make irq_chip immutable

Kernel warns about mutable irq_chips:
"not an immutable chip, please consider fixing!"

Flag the irq_chip as IRQCHIP_IMMUTABLE, add the new helper functions,
and call the appropriate gpiolib functions.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20250509-gpio-v1-4-639377c98288@nxp.com


Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>