net: dsa: bcm_sf2: request and handle clocks

Fetch the corresponding clock resource and enable/disable it during
suspend/resume if and only if we have no ports defined for Wake-on-LAN.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>