board: mvebu: drop unused ETH_PHY macro definitions

These macros are not used anywhere in the boards code.

Cc: Chris Packham <chris.packham@alliedtelesis.co.nz>
Cc: Dirk Eibach <dirk.eibach@gdsys.cc>
Cc: Mario Six <mario.six@gdsys.cc>
Cc: Dennis Gilmore <dgilmore@redhat.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Reviewed-by: Stefan Roese <sr@denx.de>
Signed-off-by: Stefan Roese <sr@denx.de>