gpio: zynq: fix zynqmp_gpio not an immutable chip warning

Make the struct irq_chip const and flag it as IRQCHIP_IMMUTABLE,
call gpiochip_disable_irq() in the .irq_mask() callback and
gpiochip_enable_irq() in the .irq_unmask() callback to fix
"gpio gpiochip1: (zynqmp_gpio): not an immutable chip" warning.

Signed-off-by: Manikanta Guntupalli <manikanta.guntupalli@amd.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>