pwm: microchip-core: Make use of devm_pwmchip_alloc() function

This prepares the pwm-microchip-core 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.

Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/c6660b655c1d4c9d79280030e24090a4a4c5dd8b.1707900770.git.u.kleine-koenig@pengutronix.de


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