arm: kirkwood: Dreamplug: Use Ethernet PHY name and address from device tree

In DM Ethernet, the old "egiga0" and 'egiga1" names are no longer valid,
so replace these with Ethernet PHY names from device tree. Also, read
Ethernet PHY address for each port from device tree.

Signed-off-by: Tony Dinh <mibodhi@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>