net: zynq_gem: Add support for SGMII interface

Add support of SGMII interface for zynq GEM.
Read xlnx,emio property from DT.

Signed-off-by: Siva Durga Prasad Paladugu <sivadur@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>