dt-bindings: usb: dwc2: document the vbus-supply property

Various boards have an external VBUS supply regulator. This regulator
depends on the current mode of the controller which is defined as:
- dr_mode set to either "host" or "peripheral" (fixed value)
- dr_mode set to "otg", based on the OTG status the dwc2 controller
  internally switches between "host" and "peripheral" mode (selection
  happens at runtime)

Based on the current mode the regulator has to be enabled or disabled:
- host: provide power to the connected USB device, thus the regulator
  has to be enabled
- peripheral: the host device to which the controller is connected
  provides power, thus the regulator has to be disabled

Add the dt-bindings documentation for this property so .dts authors know
that this property exists and how it behaves.

Fixes: 531ef5eb ("usb: dwc2: add support for host mode external vbus supply")
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>