net: stmmac: dwmac-rk: add support for rk3308 gmac

The Rockchip RK3308 SoC has a gmac with only the RMII interface signals
exposed.
This patch adds support for it.

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