ixgbe: Add definitions for x550em_a 10G MAC

Add definitions for a x550em_a 10G MAC device with a native SFP
interface.

Signed-off-by: Mark Rustad <mark.d.rustad@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>