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.

Fixes: 501c574f ("cpufreq: mediatek: Add support of cpufreq to MT2701/MT7623 SoC")
Signed-off-by: Rex-BC Chen <rex-bc.chen@mediatek.com>
[ Viresh: Commit log and Subject ]
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>