platform/x86: asus_wmi: Support throttle thermal policy

Throttle thermal policy ACPI device is used to control CPU cooling and
throttling. This patch adds sysfs entry for setting current mode and
Fn+F5 hotkey that switches to next.

Policy modes:
* 0x00 - default
* 0x01 - overboost
* 0x02 - silent

Signed-off-by: Leonid Maksymchuk <leonmaxx@gmail.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>