net: stmmac: socfpga: add RMII phy mode

Add option for enabling RMII phy mode.

Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Wei Liang Lim <wei.liang.lim@intel.com>
Signed-off-by: Dinh Nguyen <dinguyen@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>