pwm: imx: Remove enabled field from struct imx_chip

We can test PWMF_ENABLED bit to know if pwm is enabled or not.
Thus remove enabled field from struct imx_chip.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>