phy: sun4i-usb: Do not drive VBUS with external VBUS present

It is possible to use host-side USB with externally-provided VBUS. For
example, some USB OTG cables have an extra power input which powers
both the board and the USB peripheral.

To support this setup, skip enabling the VBUS switch/regulator if VBUS
voltage is already present. This behavior matches the Linux PHY driver.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>