thermal/governors: Prefix all source files with gov_

Bang-bang governor source file is prefixed with gov_. Do the same for
other governors for consistency so they're easy to find in the sources.

Signed-off-by: Amit Kucheria <amit.kucheria@linaro.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/b9a85d3204712f14e320504948c12712dc0b291b.1589199124.git.amit.kucheria@linaro.org