dm: power: Add regulator flags to centralise auto-set logic

Decide when the regulator is set up whether we want to auto-set the voltage
or current. This avoids the complex logic spilling into the processing code.

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