dt-bindings: soc: socionext: Add UniPhier DWC3 USB glue layer

Add DT binding schema for components belonging to the platform-specific
DWC3 USB glue layer implemented in UniPhier SoCs.

This USB glue layer works as a sideband logic for the host controller,
including core reset, vbus control, PHYs, and some signals to the
controller.

Signed-off-by: Kunihiko Hayashi <hayashi.kunihiko@socionext.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20221213082449.2721-17-hayashi.kunihiko@socionext.com
Signed-off-by: Rob Herring <robh@kernel.org>