MFC r345855: cpsw: use `phy-handle` in FDT to find PHY address

MFC r345855: cpsw: use `phy-handle` in FDT to find PHY address

In r337703 DTS files were updated to Linux 4.18, including Linux commit
4d8b032d3c03f4e9788a18bbb51b10e6c9e8a56b which removed the `phy_id`
property from am335x-bone-common (as the property was deprecated).

Use `phy-handle` via fdt_get_phyaddr, keeping the existing code as a
fallback for old DTBs.

PR:		236624
Submitted by:	manu, Gerald Aryeetey <aryeeteygerald_rogers.com>
Reported by:	Gerald Aryeetey
Reviewed by:	manu
Sponsored by:	The FreeBSD Foundation