devicetree: bindings: Add optional dynamic-power-coefficient property

The dynamic power consumption of a device is proportional to the
square of voltage (V) and the clock frequency (f). It can be expressed as

Pdyn = dynamic-power-coefficient * V^2 * f.

The coefficient represents the running time dynamic power consumption in
units of mw/MHz/uVolt^2 and can be used in the above formula to
calculate the dynamic power in mW.

Signed-off-by: Punit Agrawal <punit.agrawal@arm.com>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>