gpio: Add the devm_fwnode_get_index_gpiod_from_child() helper

devm_fwnode_get_gpiod_from_child() currently allows GPIO users to
request a GPIO that is defined in a child fwnode instead of directly in
the device fwnode.
Extend this API by adding the devm_fwnode_get_index_gpiod_from_child()
helper which does the same except you can also specify an index in case
the 'xx-gpios' property describe several GPIOs.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>