pwm: renesas-tpu: Make use of devm_pwmchip_alloc() function

pwm: renesas-tpu: Make use of devm_pwmchip_alloc() function

This prepares the pwm-renesas-tpu 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.

Also convert the to_tpu_device() helper macro to a static inline to get
some type safety.

Link: https://lore.kernel.org/r/aac4a9546b0f474d991144aa925c83cfa7abe2c0.1707900770.git.u.kleine-koenig@pengutronix.de
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>