cpufreq: qcom-hw: provide online/offline operations

Provide lightweight online and offline operations. This saves us from
parsing and tearing down the OPP tables each time the CPU is put online
or offline.

Tested-by: Vladimir Zapolskiy <vladimir.zapolskiy@linaro.org>
Reviewed-by: Vladimir Zapolskiy <vladimir.zapolskiy@linaro.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>