cpufreq: interactive: Permanently cache tunable values

Userspace might change tunable values for a governor. Currently, if
all CPUs in a policy go offline, governor frees its tunable. This
wipes out all userspace modifications. Kernel drivers can call
cpu_up/down() directly and thus userspace won't have a chance to
restore the tunables.

Permanently save tunable struct in a per_cpu field so that we
preserve tunable values across hotplug, suspend/resume and governor
switch.

Change-Id: I126b8278c8e75c8eadb3e2ddfe97fcc72cddfa23
Signed-off-by: Junjie Wu <junjiew@codeaurora.org>