cpufreq: mediatek: Unregister platform device on exit

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>