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
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D19814