ixgbe: Add support for x550em_a 10G MAC type

Add support for x550em_a 10G MAC type to the ixgbe driver. The new
MAC includes new firmware commands that need to be used to control
PHY and IOSF access, so that support is also added. The interface
supported is 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>