pwm: stm32-lp: Make use of devm_pwmchip_alloc() function

This prepares the pwm-stm32-lp 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.

Link: https://lore.kernel.org/r/04af7b3d00bc932dd025200a3bf74527c29ca47a.1707900770.git.u.kleine-koenig@pengutronix.de


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