cpufreq: Remove cpufreq_generic_attrs

All users of cpufreq_generic_attr are migrated now, remove it. While at
it, also stop exporting attributes for available and boost frequencies
as they are only used by cpufreq core now.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Acked-by: Rafael J. Wysocki <rafael@kernel.org>