ixgbe: add debugfs support

This patch adds debugfs support to the ixgbe driver to give
users the ability to access kernel information and to
simulate kernel events.

The filesystem is set up in the following driver/PCI-instance
hierarchy:
<debugfs>
   |-- ixgbe
	|-- PCI instance
	|	|-- attribute files

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>