pwm: cros-ec: Make use of devm_pwmchip_alloc() function

pwm: cros-ec: Make use of devm_pwmchip_alloc() function

This prepares the pwm-cros-ec 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.
The probe function had to be changed a bit because the number of PWMs
must be determined before allocation of the pwm_chip and its private
data now.

Reviewed-by: Tzung-Bi Shih <tzungbi@kernel.org>
Link: https://lore.kernel.org/r/b0afe9e25340f0a274ff3806687cf37a6cc31557.1707900770.git.u.kleine-koenig@pengutronix.de
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>