dt-bindings: net: rockchip-dwmac: add rv1126 compatible

Add compatible string for RV1126 gmac, and constrain it to
be compatible with Synopsys dwmac 4.20a.

Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Jagan Teki <jagan@edgeble.ai>
Signed-off-by: Anand Moon <anand@edgeble.ai>
Link: https://lore.kernel.org/r/20220920140944.2535-1-anand@edgeble.ai


Signed-off-by: Jakub Kicinski <kuba@kernel.org>