Instead of passing a global GPIO number for the enable GPIO, pass
a descriptor looked up with the standard devm_gpiod_get_optional()
call.

All users of this regulator use device tree so the transition is
pretty smooth.
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>