ACPI and power management fixes for 3.12-rc3

 1) After recent locking changes in the cpufreq core it is possible
    to trigger BUG_ON(!policy) in lock_policy_rwsem_read() if
    cpufreq_get() is called before registering a cpufreq driver.
    Fix from Viresh Kumar.

 2) If intel_pstate has been loaded already, it doesn't make sense
    to do anything in acpi_cpufreq_init() and moreover doing something
    in there in that case may be harmful, so make that function return
    immediately if another cpufreq driver is already present.  From
    Yinghai Lu.

 3) The ACPI IPMI driver sometimes attempts to acquire a mutex from
    interrupt context, which can be avoided by replacing that mutex
    with a spinlock.  From Lv Zheng.

 4) A NULL pointer may be dereferenced by the exynos5440 cpufreq
    driver if a memory allocation made by it fails.  Fix from
    Sachin Kamat.

 5) Hanjun Guo's commit fixes a typo in the kerneldoc comment
    documenting acpi_bus_unregister_driver().

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