net: phy: ti: Add binding for the CLK_OUT pin muxing

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.

Based on commit 9708fb630d19 ("net: phy: dp83867: Add binding for
the CLK_OUT pin muxing option") of mainline linux kernel.

Signed-off-by: Janine Hagemann <j.hagemann@phytec.de>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>