net: dsa: bcm_sf2: add suspend/resume callbacks

Implement the suspend/resume callbacks for the Broadcom Starfighter 2
switch driver. Suspending the switch requires masking interrupts and
shutting down ports. Resuming the switch requires a software reset since
we do not know which power-sate we might be coming from, and re-enabling
the physical ports that are used.

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