pwm: clk: Make use of devm_pwmchip_alloc() function

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

Link: https://lore.kernel.org/r/c02ccc955fe7a0aec7ce0ccaf5cd2bd902ae687f.1707900770.git.u.kleine-koenig@pengutronix.de


Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>