power: supply: bq25890: Add get_voltage support to Vbus regulator

The chip is capable of reporting Vbus voltage, add .get_voltage
implementation to Vbus regulator to report current Vbus voltage.
This requires for the Vbus regulator to be registered always
instead of the current state where the regulator is registered
only in case USB PHY is not found.

Do not provide Vbus regulator enable/disable ops in case USB PHY
is present, as they would race with USB PHY notifier which is also
used to toggle OTG boost mode.

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