pwm: imx27: Make use of devm_pwmchip_alloc() function

pwm: imx27: Make use of devm_pwmchip_alloc() function

This prepares the pwm-imx27 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_imx27_chip() helper macro to a static inline to
get some type safety.

Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Link: https://lore.kernel.org/r/3da748c80e9bbe29e7c119190dbc4bd67d159f22.1707900770.git.u.kleine-koenig@pengutronix.de
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>