pwm: fsl-ftm: set pwm_chip can_sleep flag

The implementation of .config(), .enable() and .disable() operations in this
driver may sleep, thus set pwm_chip can_sleep flag.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Xiubo Li <Li.Xiubo@freescale.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>