ixgbe: added reg_ops file to debugfs

Added the reg_ops file to debugfs with commands to read and write
a register to give users the ability to read and write individual
registers on the fly.

Signed-off-by: Catherine Sullivan <catherine.sullivan@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>