dt-bindings: net: ti,dp83869: Drop value on boolean 'ti,max-output-impedance'

DT booleans don't have a value and 'ti,max-output-impedance' is defined and
used as a boolean. So drop the bogus value in the example.

Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220107030513.2385482-1-robh@kernel.org