net: zynq_gem: Dont enable SGMII and PCS selection

Dont enable SGMII and PCS selection if internal PCS/PMA
is not used, by getting the info about internal/external
PCS/PMA usage from dt property "is-internal-phy".

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