net: bcmgenet: hook into the build system

This patch adds a new configuration symbol: CONFIG_BCMGENET which allows
us to build the Broadcom GENET driver and hook the driver files into the
build system.

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