PM / devfreq: Add devfreq driver for simple device

This driver registers a devfreq device that allows devfreq governors to
scale the frequency of a device clock. This single driver can be used to
support multiple devices as long as those devices don't have a need or
mechanism to monitor their load and use clock APIs to control their
device/core clock.

If devices need to support device specific status monitoring, they could
extend this driver to allow registering device specific status monitoring
functions or write their own specific devfreq device driver.

Change-Id: Ie1797acf7b35cac6dc49428e270c23082634eb1e
Signed-off-by: Saravana Kannan <skannan@codeaurora.org>
Signed-off-by: Rama Aparna Mallavarapu <aparnam@codeaurora.org>