gpiolib: Introduce gpiochip_irqchip_add_domain()

The function connects an IRQ domain to a gpiochip and reuses
gpiochip_to_irq() which is provided by gpiolib.

gpiochip_irqchip_* and regmap_irq partially provide the same
functionality. This function will help to connect just the
minimal functionality of the gpiochip_irqchip which is needed to
work together with regmap-irq.

Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20200528145845.31436-2-michael@walle.cc


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>