net: zynq_gem: Add SGMII support for zynqMP

PCS auto negotaiation bit should be enabled
along with SGMII autonegotation enabled
in phy.

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