The irq/pin mapping is used to lookup the pin to mux to the irq
function when the irq is enabled. It is created when gpio_to_irq
is called. Creating the mapping during init allows us to map the
interrupts directly from the device tree.

Originally the IRQ to pin mapping was created when gpio_to_irq
was called with a GPIO handle. The mapping in turn is used to mux
the pin into EINT mode.

If the mapping is created during gpio_to_irq, we can't use the
interrupts directly, i.e. through the DT with "interrupts = <&pio A 4>".

Instead we'd have to use "gpios = <&pio A B>", then pass the gpio
through to gpio_to_irq.
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>