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

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