dt-bindings: net: rockchip-dwmac: add rk3308 gmac compatible

The Rockchip RK3308 has a gmac that is not fully compatible with any of the
other Rockchip gmacs.
This patch adds a compatible string for it.

Signed-off-by: Tobias Schramm <t.schramm@manjaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>