ixgbe: add ipsec register access routines

Add a few routines to make access to the ipsec registers just a little
easier, and throw in the beginnings of an initialization.

Signed-off-by: Shannon Nelson <shannon.nelson@oracle.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>