thermal: brcmstb_thermal: Prepare to support a different process

The driver is currently assuming that it is operating with a 28nm
process chip, which has a specific formula to convert temperature to a
code and vice versa. Update the code to support providing two key
values: offset and multiplier to derive the correct formulas.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Amit Kucheria <amit.kucheria@linaro.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20200114190607.29339-3-f.fainelli@gmail.com