dm: power: Don't return an error when regulators are not autoset

Not all regulators can be set up automatically. Adjust the code so that
regulators_enable_boot_on() will return success when some are skipped.
Only genuine errors are reported.

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