platform/x86: asus-wmi: Add support for charge threshold

Most newer ASUS laptops supports limiting the battery charge level, which
help prolonging the battery life.

Tested on a Zenbook UX430UNR.

Signed-off-by: Kristian Klausen <kristian@klausen.dk>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>