igb: Add function and pointers for 82580 low power state settings.

82580 and later parts did not have low power setting functions.  This patch
adds the specific functions, pointers and assignments for these low
power settings.

Signed-off-by: Carolyn Wyborny <carolyn.wyborny@intel.com>
Tested-by: Jeff Pieper <jeffrey.e.pieper@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>