pwm: Fix compilation error when CONFIG_PWM is not defined

Add dummy implemention of public symbols for compilation-safe inclusion
of include/linux/pwm.h file when CONFIG_PWM is not defined.

Reported-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Tushar Behera <tushar.behera@linaro.org>
Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>