net/macb: add DT support for Cadence macb/gem driver

Allow the device tree to provide the mac address and the phy mode.

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
[nicolas.ferre@atmel.com: change "compatible" node property, doc and DT hwaddr]
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
[jamie@jamieiles.com: add "gem" compatibility strings and doc]
Acked-by: Jamie <Iles&lt;jamie@jamieiles.com>
Acked-by: David S. Miller <davem@davemloft.net>