power: regulator: Add support for gpio regulators

Add support for gpio regulators. As of now this driver caters
to gpio regulators with one gpio. Supports setting voltage values to gpio
regulators and retrieving the values.

Signed-off-by: Keerthy <j-keerthy@ti.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

include/power/regulator.h

@@ -108,6 +108,7 @@ enum regulator_type {
 	REGULATOR_TYPE_BUCK,
 	REGULATOR_TYPE_DVS,
 	REGULATOR_TYPE_FIXED,
+	REGULATOR_TYPE_GPIO,
 	REGULATOR_TYPE_OTHER,
 };