Traditionally each PWM device had a unique ID stored in the "pwm" member
of struct pwm_device. However this number was hardly used and dropped
in the previous commit. To identify a certain PWM you're supposed to use
the chip's ID and the hwpwm of the PWM device now.

With the PWM chip base gone PWM chips can get their IDs better and
simpler using an idr.

This is expected to change the numbering of PWM chips, but nothing
should rely on the numbering anyhow.

Other than that the side effects are:

 - The PWM chip IDs are smaller and in most cases consecutive.
 - The ordering in /sys/kernel/debug/pwm is ordered by ascending PWM
   chip ID.
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>