pwm: Add devm_of_pwm_get() as exported API for users

When booted with DT users can use devm version of of_pwm_get() to benefit
from automatic resource release.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Acked-by: Thierry Reding <thierry.reding@avionic-design.de>
Signed-off-by: Bryan Wu <cooloney@gmail.com>