samples/drivers: led_pwm: Fixes for device testing

Modify led_pwm sample in order to make it usable in device testing:
- Remove parenthesis from the logs as they prevent use of regex
- Add a harness_config multiline regex in order to enable verdict
generation
- Update README

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>
3 files changed