dt-bindings: net: Document Broadcom SF2 switch clocks

Describe the two possible clocks feeding into the Broadcom SF2
integrated Ethernet switch. BCM7445 systems have two clocks, one for the
main switch core clock, and another for controlling the switch clock
divider whereas BCM7278 systems only have the first kind.

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>