dt-bindings: pwm: adi,axi-pwmgen: Increase #pwm-cells to 3

Using 3 cells allows to pass additional flags and is the normal
abstraction for new PWM descriptions. There are no device trees yet to
adapt to this change.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Trevor Gamblin <tgamblin@baylibre.com>
Link: https://lore.kernel.org/r/20241024102554.711689-2-u.kleine-koenig@baylibre.com


Signed-off-by: Uwe Kleine-König <ukleinek@kernel.org>