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/Makefile b/drivers/power/regulator/Makefile
index f9c4e6d..cc8326d 100644
--- a/drivers/power/regulator/Makefile
+++ b/drivers/power/regulator/Makefile
@@ -7,3 +7,4 @@
 
 obj-$(CONFIG_DM_REGULATOR) += regulator-uclass.o
 obj-$(CONFIG_DM_REGULATOR_MAX77686) += max77686.o
+obj-$(CONFIG_DM_REGULATOR_FIXED) += fixed.o