dt-bindings: cpufreq: move MediaTek cpufreq dt-bindings document to proper place

The old place is Documentation/devicetree/bindings/clock/ that would
let people hard to find how to use MediaTek cpufreq driver, so moving
it to the appropriate place as other cpufreq drivers done would be
better.

Signed-off-by: Sean Wang <sean.wang@mediatek.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>