pwm: raspberrypi-poe: Make use of devm_pwmchip_alloc() function

This prepares the pwm-raspberrypi-poe 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/4654a6438ab618574afc3bb73bf402df7c254ee2.1707900770.git.u.kleine-koenig@pengutronix.de


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