drivers: pwm_shim: correct api argument inconsistency

Correct the argument definition inconsistency for the set
value API. To be consistent with other pwm sub drivers, the
on and off arguments for the set value api are re-defined.

On: how far (number of timer count) from the beginning of a PWM
    cycle the PWM pin will be asserted.
Off:how far (number of timer count) from the beginning of a PWM
    cycle the PWM pin will be de-asserted.

Jira: ZEP-642

Change-Id: I7b39f98f1935fc3499fb36dd813abed62b86c1e7
Signed-off-by: Baohong Liu <baohong.liu@intel.com>