net: phy: dp83867: Add documentation for SGMII mode type

Add documentation of ti,sgmii-ref-clock-output-enable
which can be used to select SGMII mode type (4 or 6-wire).

Signed-off-by: Vitaly Gaiduk <vitaly.gaiduk@cloudbear.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>