pwm: imx: Rewrite v1 code to facilitate switch to atomic PWM

The code has been rewritten to remove "generic" calls to
imx_pwm_{enable|disable|config}.

Such approach would facilitate switch to atomic PWM (a.k.a ->apply())
implementation.

Suggested-by: Stefan Agner <stefan@agner.ch>
Suggested-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Reviewed-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Lukasz Majewski <l.majewski@majess.pl>
Reviewed-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>