power: supply: bq25890: Factor out regulator registration code

Pull the regulator registration code into separate function, so it can
be extended to register more regulators later. Currently this is only
moving ifdeffery into one place and other preparatory changes. The
dev_err_probe() output string is changed to explicitly list vbus
regulator failure, so that once more regulators are registered, it
would be clear which one failed.

Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>