dt-bindings: phy: dp83867: Add documentation for disabling clock output

Based on commit 980066e6d964 ("dt-bindings: phy: dp83867: Add documentation
for disabling clock output") of mainline linux kernel.

The clock output is generally only used for testing and development and not
used to daisy-chain PHYs.  It's just a source of RF noise afterward.

Add a mux value for "off".  I've added it as another enumeration to the
output property.  In the actual PHY, the mux and the output enable are
independently controllable.  However, it doesn't seem useful to be able
to describe the mux setting when the output is disabled.

Document that PHY's default setting will be left as is if the property
is omitted.

Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>