thermal: imx: improve comments describing algorithm for temp calculation

The description of the implemented algorithm is hardly understandable
without having the right application note side-by-side to the code.

Fix this by using shorter and more intuitive variable names, describe
their meaning and transform a single formula instead of first talking about
slope and then about "milli_Tmeas".

There are no code changes.

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>