pwm: lpc18xx-sct: Make use of devm_pwmchip_alloc() function

This prepares the pwm-lpc18xx-sct 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/d480891340e927705843cd09e5777d4daaa6c9cc.1707900770.git.u.kleine-koenig@pengutronix.de


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