regulator: gpio: Simplify probe path

Use devm_* managed device resources and create a local
struct device *dev variable to simplify the code inside
probe().

Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>