dm: regulator: add fixed voltage regulator driver

This driver implements regulator operations for fixed Voltage/Current
value regulators. beside the standard regulator constraints, which are
put into the uclass platform data, a typical fixed regulator node provides
few additional properties like:
- gpio
- gpio-open-drain
- enable-active-high
- startup-delay-us
The only 'gpio' is used by this driver and is kept in structure of type
'fixed_regulator_platdata', as a device platform data (dev->platdata).

The driver implements:
- get_value
- get_current
- get_enable
- set_enable

The regulator calls and commands can be used for fixed-regulator devices,
and the proper error will be returned for prohibited.

Signed-off-by: Przemyslaw Marczak <p.marczak@samsung.com>
Acked-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/power/regulator/Kconfig b/drivers/power/regulator/Kconfig
index 0cdfabc..54ce188 100644
--- a/drivers/power/regulator/Kconfig
+++ b/drivers/power/regulator/Kconfig
@@ -23,3 +23,11 @@
 	This config enables implementation of driver-model regulator uclass
 	features for REGULATOR MAX77686. The driver implements get/set api for:
 	value, enable and mode.
+
+config DM_REGULATOR_FIXED
+	bool "Enable Driver Model for REGULATOR Fixed value"
+	depends on DM_REGULATOR
+	---help---
+	This config enables implementation of driver-model regulator uclass
+	features for fixed value regulators. The driver implements get/set api
+	for enable and get only for voltage value.