pwm: Add different PWM output types support

Normally, PWM channel has fixed output until software request to change
its settings. There are some PWM devices which their outputs could be
changed autonomously according to a predefined pattern programmed in
hardware. Add pwm_output_type enum type to identify these two different
PWM types and add relevant helper functions to set and get PWM output
types and pattern.

Change-Id: Ia1f914a45ab4f4dd7be037a395eeb89d0e65a80e
Signed-off-by: Fenglin Wu <fenglinw@codeaurora.org>