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>