dt-bindings: net: snps,dwmac: Add dwmac-5.20 version

Add dwmac-5.20 IP version to snps.dwmac.yaml

Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Emil Renner Berthing <kernel@esmil.dk>
Signed-off-by: Samin Guo <samin.guo@starfivetech.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>