cpufreq: cppc: Set policy->boost_supported

cpufreq: cppc: Set policy->boost_supported

With a later commit, the cpufreq core will call the ->set_boost()
callback only if the policy supports boost frequency. The
boost_supported flag is set by the cpufreq core if policy->freq_table is
set and one or more boost frequencies are present.

For other drivers, the flag must be set explicitly.

With this, the local variable boost_supported isn't required anymore.

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