net: phy: xilinx: Be compatible with live OF tree

Following the same updates that were done to the fixed phy driver,
use ofnode_ APIs instead of fdt_ APIs so that the Xilinx PHY driver
can support live DT.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>