ARM: sun7i: a20: cubietruck: Tie AXP209's USB power supply to USB PHY

The USB PHY can use either a GPIO pin or the PMIC's USB power supply
to sense VBUS. Since both options are available on the Cubietruck,
add the missing property for the USB power supply to the USB PHY node.

The device tree provides all usable options. Ultimately, which method
is used is up to the driver implementation.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>