gpio: provide for_each_hwgpio()

We only provide iterators for requested GPIOs to provider drivers. In
order to allow them to display debug information about all GPIOs, let's
provide a variant for iterating over all GPIOs.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>