phy-sun4i-usb: Add support for monitoring vbus via a power-supply

On some boards there is no vbus_det gpio pin, instead vbus-detection for
otg can be done via the pmic.

This commit adds support for monitoring vbus_det via the power_supply
exported by the pmic, enabling support for otg on these boards.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>