pwm: vt8500: Make use of devm_pwmchip_alloc() function

pwm: vt8500: Make use of devm_pwmchip_alloc() function

This prepares the pwm-vt8500 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_vt8500_chip() helper macro to a static inline to
get some type safety.

Link: https://lore.kernel.org/r/b203c4448db23ebad1165b7bce43ac41468c4e89.1707900770.git.u.kleine-koenig@pengutronix.de
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>