Documentation: add Device tree bindings for Broadcom GENET

This patch adds the Device Tree bindings for the Broadcom GENET Gigabit
Ethernet controller. A bunch of examples are provided to illustrate the
versatile aspect of the hardare.

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