net: stmmac: dwmac-rk: Add integrated PHY support for rk3228

There is only one mac controller in rk3228, which could connect to
external PHY or integrated PHY, use the grf_com_mux bit15 to route
external/integrated PHY.

Signed-off-by: David Wu <david.wu@rock-chips.com>
Signed-off-by: David S. Miller <davem@davemloft.net>