phy: sun4i-usb: Determine VBUS detection pin from devicetree

phy: sun4i-usb: Determine VBUS detection pin from devicetree

So far Allwinner boards controlled the USB VBUS detection via the
respective GPIO pin specified in Kconfig, as a string. All boards should
have the same GPIO already specified in the devicetree, in the
usb0_vbus_det-gpios property.

Convert the usage of the Kconfig configured GPIO over to query that
information from the devicetree, then use the existing DM GPIO
infrastructure to request the GPIO.
Only PHY0 supports USB-OTG, so limit the GPIO request to that PHY, to
avoid claiming it multiple times.

This removes the need to name that GPIO in the defconfig file.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Jernej Skrabec <jernej.skrabec@gmail.com>