This prepares the pwm-imx1 driver to further changes of the pwm core
outlined in the commit introducing devm_pwmchip_alloc(). There is no
intended semantical change and the driver should behave as before.

Also convert the to_pwm_imx1_chip() helper macro to a static inline to
get some type safety.

Link: https://lore.kernel.org/r/c6995e0d281255a7514edf9f25a561302dcaf65f.1707900770.git.u.kleine-koenig@pengutronix.deSigned-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>