igb: Add support functions to access thermal data.

Some of our devices have internal sensors for reporting thermal data.
This patch creates the interface to the sensors for exporting via sysfs.
Subsequent patch will actually export the data.

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>