gpio: Introduce struct gpio_irq_chip

This new structure will be used to group all fields related to interrupt
handling in a GPIO chip. Doing so will properly namespace these fields
and make it easier to distinguish which fields are used for IRQ support.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Acked-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>