pwm: mediatek: Make use of devm_pwmchip_alloc() function

This prepares the pwm-mediatek 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/513b2037c00ebf2f3a69472f4b99adaf4fb40d83.1707900770.git.u.kleine-koenig@pengutronix.de


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