platform/x86: asus-wmi: Add userspace notification for performance mode change

When user presses Fn-F5, the driver automatically changes throttle
thermal policy (or fan boost mode, depending on laptop model).

It would be convenient for userspace software to be able to poll on
corresponding sysfs variable. For example, to show a notification about
mode change.

Note that there is currently no way to handle Fn-F5 from userspace
directly, driver does not pass it.

Signed-off-by: Vasiliy Kupriakov <rublag-ns@yandex.ru>
Link: https://lore.kernel.org/r/20200828214932.20866-2-rublag-ns@yandex.ru


Signed-off-by: Hans de Goede <hdegoede@redhat.com>