ixgbe: add hwmon interface to export thermal data

Some of our adapters have thermal data available, this patch exports
this data via hwmon sysfs interface.

Signed-off-by: Don Skidmore <donald.c.skidmore@intel.com>
Tested-by: Stephen Ko <stephen.s.ko@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>