dt-bindings: net: Add support StarFive dwmac

Add documentation to describe StarFive dwmac driver(GMAC).

Signed-off-by: Yanhong Wang <yanhong.wang@starfivetech.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Samin Guo <samin.guo@starfivetech.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>