gpiolib: Introduce gpiod_set_config()

The GPIO Aggregator will need a method to forward a .set_config() call
to its parent gpiochip.  This requires obtaining the gpio_chip and
offset for a given gpio_desc.  While gpiod_to_chip() is public,
gpio_chip_hwgpio() is not, so there is currently no method to obtain the
needed GPIO offset parameter.

Hence introduce a public gpiod_set_config() helper, which invokes the
.set_config() callback through a gpio_desc pointer, like is done for
most other gpio_chip callbacks.

Rewrite the existing gpiod_set_debounce() helper as a wrapper around
gpiod_set_config(), to avoid duplication.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20200324135653.6676-5-geert+renesas@glider.be


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