regulator: lp8788-ldo: Let core handle GPIO descriptor

Use the gpiod_get() rather than the devm_* version so that the
regulator core can handle the lifecycle of these descriptors.

Fixes: 2468f0d5

 ("regulator: lp8788-ldo: Pass descriptor instead of GPIO number")
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Signed-off-by: Mark Brown <broonie@kernel.org>