net: phy: xilinx-gmii2rgmii: Adopt clock support

Add clock support to the gmii_to_rgmii IP.
Make clk optional to keep DTB backward compatibility.

Signed-off-by: Vineeth Karumanchi <vineeth.karumanchi@amd.com>
Reviewed-by: Wojciech Drewek <wojciech.drewek@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>