ARM: orion5x: add gigabit ethernet and mvmdio device tree nodes

This patch adds mv643xx_eth and mvmdio device tree nodes for DT enabled
Orion5x boards. Phy nodes are also added with reg property set on a
per-board basis.

Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>