thermal: imx: update formula for thermal sensor

Thermal sensor used to need two calibration points which are
in fuse map to get a slope for converting thermal sensor's raw
data to real temperature in degree C. Due to the chip calibration
limitation, hardware team provides an universal formula to get
real temperature from internal thermal sensor raw data:

Slope = 0.4297157 - (0.0015976 * 25C fuse);

Update the formula, as there will be no hot point calibration
data in fuse map from now on.

Signed-off-by: Anson Huang <b20788@freescale.com>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>