pwm: sprd: Make use of devm_pwmchip_alloc() function

This prepares the pwm-sprd 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.

Tested-by: Chunyan Zhang <zhang.lyra@gmail.com>
Link: https://lore.kernel.org/r/543213f44686ee72d8f88897bf2ca616e837ae44.1707900770.git.u.kleine-koenig@pengutronix.de


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