thermal/of: Rename of-thermal.c

Core thermal framework code files should start with thermal_*.
of-thermal.c does not follow this pattern and can easily be confused
with platform driver.

Fix this by renaming it to thermal_of.c

Signed-off-by: Amit Kucheria <amit.kucheria@linaro.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/f5e233d5c5dcc7c7cb56b3448da255cb2c9ef0d1.1589199124.git.amit.kucheria@linaro.org