net: phy: Add new read ethernet phy id function

Add new function to get ethernet phy id from compatible property
of the mdio phy node.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: T Karthik Reddy <t.karthik.reddy@xilinx.com>
Link: https://lore.kernel.org/r/16019efb3820a50330935fdaae191cec1f101b5c.1645627539.git.michal.simek@xilinx.com