pwm: imx: Let PWM be active during suspend

On a imx6q-cubox-i board, which has an LED driven by PWM, when the system
goes into suspend the PWM block is disabled by default, then the PWM pin
goes to logic level zero and turn on the LED during suspend, which is not
really the behaviour we want to see.

By keeping the PWM enabled during suspend via STOPEN bit, the pwm-leds
driver sets the brightness to zero in suspend and then the LED is
turned off as expected.

So always set the STOPEN to fix the PWM behaviour in suspend.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>