net: phy: dp83867: Add SGMII helper for configuration

net: phy: dp83867: Add SGMII helper for configuration

The code assumed that if the interface is not RGMII configured
then it must be SGMII configured.  This device has the ability
to support most of the MII interfaces.  Therefore add the
helper for SGMII and only configure the device if the interface is
configured for SGMII.

Signed-off-by: Dan Murphy <dmurphy@ti.com>
Reviewed-by: Mugunthan V N <mugunthanvnm@ti.com>
Reviewed-by: Michal Simek <michal.simek@xilinx.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>