msm: cpufreq: Create cpufreq workqueue for freq changes
Setting frequencies may take unbounded amount of time, use private
workqueues instead of shared system workqueue.

Ondemand governor may post to the kernel workqueue, holds rwsem and
waits. This may cause msm_cpufreq_target to repost frequency change
request to the kernel workqueue waiting on the same rwsem resulting
in watchdog reset.