dt-bindings: usb: dwc3: Add synopsys, dwc3 compatible string

dt-bindings: usb: dwc3: Add synopsys, dwc3 compatible string

The DWC USB3 driver and some DTS files like Exynos 5250, Keystone k2e, etc
expects the DWC USB3 DT node to have the compatible string with the
"synopsys" vendor prefix. Let's add the corresponding compatible string to
the controller DT schema, but mark it as deprecated seeing the Synopsys,
Inc. is presented with just "snps" vendor prefix.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Link: https://lore.kernel.org/r/20201210090944.16283-13-Sergey.Semin@baikalelectronics.ru
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>