dt-bindings: pwm: Add Apple PWM controller

Apple SoCs such as the M1 contain a PWM controller used
among other things to control the keyboard backlight.

Signed-off-by: Sasha Finkelstein <fnkl.kernel@gmail.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Sven Peter <sven@svenpeter.dev>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>