dt-bindings: cpufreq: apple,soc-cpufreq: Add binding for Apple SoC cpufreq

This binding represents the cpufreq/DVFS hardware present in Apple SoCs.
The hardware has an independent controller per CPU cluster, and we
represent them as unique nodes in order to accurately describe the
hardware. The driver is responsible for binding them as a single cpufreq
device (in the Linux cpufreq model).

Acked-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Hector Martin <marcan@marcan.st>