thermal/debugfs: Print mitigation timestamp value in milliseconds

thermal/debugfs: Print mitigation timestamp value in milliseconds

Because mitigation episode duration is printed in milliseconds, there
is no reason to print timestamp information for mitigation episodes in
smaller units which also makes it somewhat harder to interpret the
numbers.

Print it in milliseconds for consistency.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>