Use PHY API phy_get_ofnode() helper to get PHY DT node.

Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
Tested-by: Siva Durga Prasad Paladugu <siva.durga.paladugu@xilinx.com>