dt-bindings: dma: apple,admac: Add reset

On the SoCs there is usually a shared audio reset line, so add
a property for that.

Signed-off-by: Martin Povišer <povik+lin@cutebit.org>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Hector Martin <marcan@marcan.st>