ixgbe: add support functions to access thermal data

Some 82599 adapters contain thermal data that we can get to via
an i2c interface.  These functions provide support to get at that
data.  A following patch will export this data.

Signed-off-by: Don Skidmore <donald.c.skidmore@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>