regulator: gpio-regulator: Allow use of GPIO controlled regulators though DT

Here we provide the GPIO Regulator driver with Device Tree capability, so
that when a platform is booting with DT instead of platform data we can
still make full use of it.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>