dt-bindings: usb: Convert DWC2 bindings to json-schema

Convert DWC2 bindings to DT schema format using json-schema.
DWC2 is widely use but a couple of compatibles and properties
(vusb_d-supply,vusb_a-supply) were missing in dwc2.txt, the
patch add them.

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@st.com>
Signed-off-by: Rob Herring <robh@kernel.org>