thermal: imx: update to new formula according to NXP AN5215

According to an application note from 03/2017 there is an updated formula to
calculate the temperature that better matches reality. This is implemented here.

While updating move the magic constants from cpp defines which are far above the
explaining formula to constants in the code just under the explaining comment.

Reviewed-by: Leonard Crestez <leonard.crestez@nxp.com>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>