Thermal: imx: correct critical trip temperature setting

On latest i.MX6 SOC with thermal calibration data of 0x5A100000,
the critical trip temperature will be an invalid value and
cause system auto shutdown as below log:

thermal thermal_zone0: critical temperature reached(42 C),shutting down

So, with universal formula for thermal sensor, only room
temperature point is calibrated, which means the calibration
data read from fuse only has valid data of bit [31:20], others
are all 0, the critical trip point temperature can NOT depend
on the hot point calibration data, here we set it to 20 C higher
than default passive temperature.

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>