net: bcmgenet: add driver definitions and private structure

This patchs adds the bcmgenet.h header file which contains all the
hardware definitions for the GENETv1 to v4 hardware blocks as well as
the driver private structure and MIB counters.
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>