net: bcmgenet: add main driver file

This patch adds the BCMGENET main driver file which supports the
following:

- GENET hardware from V1 to V4
- support for reading the UniMAC MIB counters statistics
- support for the 5 transmit queues
- support for RX/TX checksum offload and SG

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