[BACKPORT]: cpufreq: Fix governor start/stop race condition

Cpufreq governor's stop and start operation should be kept in sequence.
If not, there will be unexpected behavior, for example:

There are 4 CPUs and policy->cpu=cpu0, cpu1/2/3 are linked to cpu0.
The normal sequence is as below:

1) Current governor is userspace, One application tries to set
governor to ondemand. It will call __cpufreq_set_policy in which it
will stop userspace governor and then start ondemand governor.

2) Current governor is userspace, Now cpu0 hotplugs in cpu3, It will
call cpufreq_add_policy_cpu in which it first stops userspace
governor, and then starts userspace governor.

Now if the sequence of above two cases interleaves, It becames
below sequence:

1) Application stops userspace governor
2) Hotplug stops userspace governor
3) Application starts ondemand governor
4) Hotplug starts a governor

In step 4, hotplug is supposed to start userspace governor, But now
the governor has been changed by application to ondemand, So hotplug
starts ondemand governor again !!!!

The solution is: Do not allow stop one policy's governor multi-times.
Governor stop should only do once for one policy, After it is stopped,
No other governor stop should be executed. also add one mutext to
protect __cpufreq_governor so governor operation can be kept in sequence.

Signed-off-by: Xiaoguang Chen <chenxg [at] marvell>

(Cherry-Picked from upstream Linux 3.11.0)

Change-Id: I09b0b88527c88ad3133b796f6eb478afd05dfdf1