Allow all the RGMII modes to be used. This would allow us to represent
the hardware better in the device tree with RGMII_ID where in most
cases the PHY's internal delay for both RX and TX are used.

Fixes: 9f93ac8d ("net-next: stmmac: Add dwmac-sun8i")
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: David S. Miller <davem@davemloft.net>