When android doing suspend/resume, we may meet the issue of
backlight is not on (pwm pin no signal) after system wakeup.
The root cause is PWM sample can not be set into the PWMSAR
register after pwm being used and disabled for a while.
The value read back after write is 0 when this issue happens.

Do a software reset after pwm disable can resolve this
issue, this makes sure the next sample update is correct.

Signed-off-by: Xinyu Chen <xinyu.chen@freescale.com>