cpsw: search for phy

I have encountered the same issue(s) on A6A boards.

I couldn't find a patch,  so I wrote this patch to update the device tree
in the davinci_mdio driver in the 3.15.1 tree, it seems to correct it. I
would welcome any input on a different approach.

https://groups.google.com/d/msg/beagleboard/9mctrG26Mc8/SRlnumt0LoMJ



v4.1-rcX: added hack around CONFIG_OF_OVERLAY
v4.2-rc3+: added if (of_machine_is_compatible("ti,am335x-bone")) so we do
not break dual ethernet am335x devices

Signed-off-by: Robert Nelson <robertcnelson@gmail.com>