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.

Patch description, dt flags have been done in mainline Linux by
commit a2111c460c0c ("net: phy: dp83867: Add documentation for SGMII mode type")
and by commit 507ddd5c0d47 ("net: phy: dp83867: Add SGMII mode type switching")

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>