net: dsa: microchip: Document property to disable reference clock

Document the new microchip,synclko-disable property which can be
specified to disable the reference clock output from the device if not
required by the board design.

Signed-off-by: Robert Hancock <robert.hancock@calian.com>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>