dt-bindings: net: rockchip-dwmac: add rk3588 gmac compatible

Add compatible string for RK3588 gmac, which is similar to the RK3568
one, but needs another syscon device for clock selection.

Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>