dm: pmic: add pmic command

This is new command for the PMIC devices based on driver model PMIC API.
Command features are unchanged:
- list UCLASS pmic devices
- show or [set] operating pmic device (NEW)
- dump registers
- read byte of register at address
- write byte to register at address

The only one change for this command is 'dev' subcommand.

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