dt-bindings: net: phy: smsc: document reference clock

Add support to specify the reference clock for the phy.

Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>