net: phy: Add phy_interface_is_sgmii to phy.h

Add a helper to phy.h to identify whether the
phy is configured for SGMII all variables.

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