net: stmmac: dwmac-sunxi: Allow all RGMII modes

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: af0bd4e9

 ("net: stmmac: sunxi platform extensions for GMAC in Allwinner A20 SoC's")
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: David S. Miller <davem@davemloft.net>