dm: regulator: add regulator command

This command is based on driver model regulator's API.
The user interface provides:
- list UCLASS regulator devices
- show or [set] operating regulator device
- print constraints info
- print operating status
- print/[set] voltage value [uV] (force)
- print/[set] current value [uA]
- print/[set] operating mode id
- enable the regulator output
- disable the regulator output

The 'force' option can be used for setting the value which exceeds
the constraints min/max limits.

Signed-off-by: Przemyslaw Marczak <p.marczak@samsung.com>
Acked-by: Simon Glass <sjg@chromium.org>