dt-bindings: net: rockchip-dwmac: add rk3568 compatible string

Add compatible string for RK3568 gmac, and constrain it to
be compatible with Synopsys dwmac 4.20a.

Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Signed-off-by: David S. Miller <davem@davemloft.net>