pwm: lpc32xx: Make use of devm_pwmchip_alloc() function

pwm: lpc32xx: Make use of devm_pwmchip_alloc() function

This prepares the pwm-lpc32xx 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_lpc32xx_pwm_chip() helper macro to a static inline
to get some type safety.

Link: https://lore.kernel.org/r/ef26238772ee9ca455a49e9b976a4f66654b26f7.1707900770.git.u.kleine-koenig@pengutronix.de
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>