regulator: max77686: switch to using fwnode_gpiod_get_index

regulator: max77686: switch to using fwnode_gpiod_get_index

gpiod_get_from_of_node() is being retired in favor of
fwnode_gpiod_get_index(), that behaves similar to gpiod_get_index(),
but can work with arbitrary firmware node. It will also be able to
support secondary software nodes.

Let's switch this driver over.

Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Link: https://lore.kernel.org/r/20191004231017.130290-8-dmitry.torokhov@gmail.com
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>