dt-bindings: cpufreq: qcom-nvmem: Document MSM8909

Document that MSM8909 is used with qcom-cpufreq-nvmem for voltage
scaling and to restrict the maximum frequency based on the speedbin
encoded in the nvmem cells.

Signed-off-by: Stephan Gerhold <stephan.gerhold@kernkonzept.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>