imx: thermal: update imx6 thermal driver according new equation

imx: thermal: update imx6 thermal driver according new equation

>From IC guys:
"
After a thorough accuracy study of the Temp sense circuit,
we found that with our current equation, an average part can
read 7 degrees lower than a known forced temperature.
We also found out that the standard variance was around 2C;
which is the tightest distribution that we could create.
We need to change the temp sense equation to center the average
part around the target temperature.
"

New equation:
Tmeas = (Nmeas - n1) / slope + t1 + offset
n1= fused room count
t1= 25
offset=3.580661
slope= 0.4148468 – 0.0015423*n1

According the new equation, update the thermal driver.
c1 and c2 changed to u64 type and update comments.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>