net: stmmac: socfpga: Allow all RGMII modes

Allow all the RGMII modes to be used.  (Not only "rgmii", "rgmii-id"
but "rgmii-txid", "rgmii-rxid")

Signed-off-by: Atsushi Nemoto <atsushi.nemoto@sord.co.jp>
Signed-off-by: David S. Miller <davem@davemloft.net>