cpufreq: Introduce policy->boost_supported flag

cpufreq: Introduce policy->boost_supported flag

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.

Add a new flag, boost_supported, which will be set to true by the
cpufreq core only if the freq table contains valid boost frequencies.

Some cpufreq drivers though don't have boost frequencies in the
freq-table, they can set this flag from their ->init() callbacks.

Once all the drivers are updated to set the flag correctly, we can check
it before enabling boost feature for a policy.

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