platform/x86: wmi: add Xiaomi WMI key driver

Some function keys on the built in keyboard on Xiaomi's notebooks does
not produce any key events when pressed in combination with the function
key. Some of these keys do report that they are being pressed via WMI
events.

This driver reports key events for Fn+F7 and double tap on Fn.

Other WMI events that are reported by the hardware but not utilized by
this driver are Caps Lock(which already work) and Fn lock/unlock.

Signed-off-by: Mattias Jacobsson <2pi@mok.nu>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>