Since recently, the kernel is nagging about mutable irq_chips:

   "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: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Bartosz Golaszewski <brgl@bgdev.pl>