net: bcmgenet: add suspend/resume callbacks

Implement suspend/resume callbacks in the GENET driver. This makes sure
that we de-initialize and re-initialize the hardware correctly before
entering suspend and when resuming.

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