net: phy: dp83867: Add documentation for CLK_OUT pin muxing

Add documentation of ti,clk-output-sel which can be used to select
a specific clock for CLK_OUT.

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