ixgbe: Add new support for X550 MAC's

This patch will add in the new MAC defines and fit it into the switch
cases throughout the driver.  New functionality and enablement support will
be added in following patches.

Signed-off-by: Don Skidmore <donald.c.skidmore@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>