We register the platform device when driver inits. However, we do not
unregister it when driver exits.
To resolve this, we declare the platform data to be a global static
variable and rename it to be "cpufreq_pdev".
With this global variable, we can do platform_device_unregister() when
driver exits.
Signed-off-by: Rex-BC Chen <rex-bc.chen@xxxxxxxxxxxx>