pwm: sl28cpld: Make use of devm_pwmchip_alloc() function

This prepares the pwm-sl28cpld 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.

Reviewed-by: Michael Walle <mwalle@kernel.org>
Link: https://lore.kernel.org/r/ee687086f7fc78264fac723f65ddc96cb7518c03.1707900770.git.u.kleine-koenig@pengutronix.de


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