net: phy: Add SGMII support for TI phy

Add support of SGMII to TI phy dp838367
Enable the SGMII and PCS settings in phy
control, CFG2 and BIST registers

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