PM / devfreq: Handle offline CPUs while creating events

If any of the CPUs monitored by a memlat device is offline while
trying to register for perf events, the registration fails
returning an error code to the governor's start_monitor function
which then fails the setting of governor and leaves the system in
a bad state.
To avoid this scenario, this patch allows for initialization errors
while registering for perf events and remembers the failing offline
CPUs. Perf event registration is tried for the failing CPUs when they
come back online. Once all the CPUs have registered for perf events
hotplug notifiers callbacks are unregistered.

Change-Id: I36c33d9dc627f60ab9d7db41913661927938e850
Signed-off-by: Rohit Gupta <rohgup@codeaurora.org>