phy: sun4i-usb: Support VBUS detection via power supply

The device tree binding provides for getting VBUS state from a device
referenced by phandle, as an optional alternative to using a GPIO. In
U-Boot, where there is no power supply class, this VBUS detection will
be implemented using a regulator device and its get_enable method.
Let's hook this up to the PHY driver.

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