net: phy: dp83867: Add SGMII mode type switching

This patch adds ability to switch beetween two PHY SGMII modes.
Some hardware, for example, FPGA IP designs may use 6-wire mode
which enables differential SGMII clock to MAC.

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