pinctrl: microchip-sgpio: make irq_chip immutable

pinctrl: microchip-sgpio: make irq_chip immutable

Since recently, the kernel is nagging about mutable irq_chips:

[    4.967050] gpio gpiochip1: (e2004190.gpio-input): not an immutable chip, please consider fixing it!

Drop the unneeded copy, flag it as IRQCHIP_IMMUTABLE, add the new
helper functions and call the appropriate gpiolib functions.

Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220428111622.1395831-1-michael@walle.cc
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>