cpufreq: Restrict enabling boost on policies with no boost frequencies

cpufreq: Restrict enabling boost on policies with no boost frequencies

It is possible to have a scenario where not all cpufreq policies support
boost frequencies. And letting sysfs (or other parts of the kernel)
enable boost feature for that policy isn't correct.

Now that all drivers (that required a change) are updated to set the
policy->boost_supported properly, check this flag before enabling boost
feature for a policy.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>