pwm: core: Remove unused devm_pwm_put()

There are no users and seems no will come of the devm_pwm_put().
Remove the function.

While at it, slightly update documentation.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>