dt: cpufreq: brcm: New binding document for brcmstb-avs-cpufreq

Add the binding document for the new brcmstb-avs-cpufreq driver.

Signed-off-by: Markus Mayer <mmayer@broadcom.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>