pwm: mxs: Make use of devm_pwmchip_alloc() function

pwm: mxs: Make use of devm_pwmchip_alloc() function

This prepares the pwm-mxs 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_mxs_pwm_chip() helper macro to a static inline to
get some type safety.

Link: https://lore.kernel.org/r/39203f3bfbf4e97654db78bdc7e0c9a2b72c1368.1707900770.git.u.kleine-koenig@pengutronix.de
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>