common: cmd regulator: command cleanup

This commit cleanups the regulator command.
The first change, is adjusting "regulator dev" command to use
"regulator-name" constraint, for setting the operating device.
Thanks to this, the regulator_get() function is removed.

This also updates do_list() function, with loop over uclass_find_*
function calls, to prevent probe of all listed regulators.

This also cleanups the printing in command.

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