dt-bindings: cpufreq: enhance MediaTek cpufreq dt-binding document

Update binding document with adding operating-points-v2 as the required
property and the cooling level as the optional properties and adding more
examples guiding people how to use MediaTek cpufreq driver for MediaTek
SoCs.

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>