gpio: document gpio irqchips a bit more, describe helpers
gpio: document gpio irqchips a bit more, describe helpers

This adds some documentation about the GPIO irqchips, what types
exist etc.

Acked-by: Alexandre Courbot <301cd6a3ea50a894b4b51e385c5eb12b9a4ffc0c@nvidia.com>
Signed-off-by: Linus Walleij <9cd9d802d23c0ed5e224beabf4ae4a5c478746ef@linaro.org>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs