regulator: max77686: Pass descriptor instead of GPIO number

Instead of passing a global GPIO number, pass a descriptor looked
up from the device tree configuration node.

Tested on Odroid U3 (with max77686 although not using any GPIOs
for regulators, so at least default paths are not broken).

Cc: Chanwoo Choi <cw00.choi@samsung.com>
Cc: Krzysztof Kozlowski <krzk@kernel.org>
Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Tested-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>