pwm: add devm_pwm_get() and devm_pwm_put()

Add resource managed variants of pwm_get() and pwm_put() for
convenience. Code is largely inspired by the equivalent devm functions
of the regulator framework.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>

Documentation/pwm.txt

@@ -36,7 +36,8 @@ Legacy users can request a PWM device using pwm_request() and free it
 after usage with pwm_free().
 
 New users should use the pwm_get() function and pass to it the consumer
-device or a consumer name. pwm_put() is used to free the PWM device.
+device or a consumer name. pwm_put() is used to free the PWM device. Managed
+variants of these functions, devm_pwm_get() and devm_pwm_put(), also exist.
 
 After being requested a PWM has to be configured using: