pwm: Add support for pwmchip devices for faster and easier userspace access

pwm: Add support for pwmchip devices for faster and easier userspace access

With this change each pwmchip defining the new-style waveform callbacks
can be accessed from userspace via a character device. Compared to the
sysfs-API this is faster and allows to pass the whole configuration in a
single ioctl allowing atomic application and thus reducing glitches.

On an STM32MP13 I see:

	root@DistroKit:~ time pwmtestperf
	real	0m 1.27s
	user	0m 0.02s
	sys	0m 1.21s
	root@DistroKit:~ rm /dev/pwmchip0
	root@DistroKit:~ time pwmtestperf
	real	0m 3.61s
	user	0m 0.27s
	sys	0m 3.26s

pwmtestperf does essentially:

	for i in 0 .. 50000:
		pwm_set_waveform(duty_length_ns=i, period_length_ns=50000, duty_offset_ns=0)

and in the presence of /dev/pwmchip0 is uses the ioctls introduced here,
without that device it uses /sys/class/pwm/pwmchip0.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Link: https://lore.kernel.org/r/ad4a4e49ae3f8ea81e23cac1ac12b338c3bf5c5b.1746010245.git.u.kleine-koenig@baylibre.com
Signed-off-by: Uwe Kleine-König <ukleinek@kernel.org>