fxos8700: Add support for temperature

The fxos8700 is primarily an accelerometer/magnetometer, but it does
also include a temperature sensor. Note that the temperature sensor is
uncalibrated and can only be used when the magnetometer is enabled
(magnetometer-only mode or hybrid mode).

Change-Id: I74c4ae68c30e0f9836caa70baed44ad8956b17ea
Signed-off-by: Maureen Helm <maureen.helm@nxp.com>