Documentation: Update the recommended pattern for GPIO irqchips

Update the documentation to get rid of the per-gpio_irq_chip
irq_chip structure, and give examples of the new pattern.

Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Reviewed-by: Bartosz Golaszewski <brgl@bgdev.pl>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220419141846.598305-11-maz@kernel.org