net: xilinx_axiemac: Fill the phy node pointer in phydev

This patch assings the phynode pointer to the phydev node as it is needed
later in the corresponding phy driver to read phy properties from DT.

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