pwm: atmel-tcb: Make use of devm_pwmchip_alloc() function

This prepares the pwm-atmel-tcb 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/c845e6c9d27c8a4037755b2ae702b0039947a3c1.1707900770.git.u.kleine-koenig@pengutronix.de


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