net: phy: dp83867: Add binding for the CLK_OUT pin muxing option

The DP83867 has a muxing option for the CLK_OUT pin. It is possible
to set CLK_OUT for different channels.
Create a binding to select a specific clock for CLK_OUT pin.

Signed-off-by: Wadim Egorov <w.egorov@phytec.de>
Signed-off-by: Daniel Schultz <d.schultz@phytec.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>