net: bcmgenet: enable driver to work without a device tree

Modify bcmgenet driver so that it can be used on Broadcom 7xxx
MIPS-based STB platforms without a device tree.

Signed-off-by: Petri Gynther <pgynther@google.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>