gpio: lpc18xx: Make irq_chip immutable

gpio: lpc18xx: Make irq_chip immutable

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

Constify lpc18xx_gpio_pin_ic, 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-v1-3-639377c98288@nxp.com
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>