dt-bindings: pwm: amlogic: Document C3 PWM

Document amlogic,c3-pwm compatible, which falls back to the meson-s4-pwm
group.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Kelvin Zhang <kelvin.zhang@amlogic.com>
Link: https://lore.kernel.org/r/20240914-c3-pwm-v2-1-ac1f34c68ac2@amlogic.com


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