arch_topology: Rename freq_scale as arch_freq_scale

Rename freq_scale to a less generic name, as it will get exported soon
for modules. Since x86 already names its own implementation of this as
arch_freq_scale, lets stick to that.

Suggested-by: Will Deacon <will@kernel.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>