pwm: omap-dmtimer: Make use of devm_pwmchip_alloc() function

This prepares the pwm-omap-dmtimer 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/78d1448b0fd1136b010f629feffb9d89c853e472.1707900770.git.u.kleine-koenig@pengutronix.de


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