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>

Documentation/devicetree/bindings/net/brcm,bcm7445-switch-v4.0.txt

@@ -50,6 +50,13 @@ Optional properties:
 - reset-names: If the "reset" property is specified, this property should have
   the value "switch" to denote the switch reset line.
 
+- clocks: when provided, the first phandle is to the switch's main clock and
+  is valid for both BCM7445 and BCM7278. The second phandle is only applicable
+  to BCM7445 and is to support dividing the switch core clock.
+
+- clock-names: when provided, the first phandle must be "sw_switch", and the
+  second must be named "sw_switch_mdiv".
+
 Port subnodes:
 
 Optional properties: