phy-sun4i-usb: Add workaround for missing Vbus det interrupts on A31

The A31 companion pmic (axp221) does not generate vbus change interrupts
when the board is driving vbus, so we must poll when using the pmic for
vbus-det _and_ we're driving vbus.

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