platform/x86: asus-wmi: Add support for SW_TABLET_MODE

On Asus 2-in-1s with a detachable keyboard the Asus WMI interface
reports if the tablet is attached to the keyboard or not.

Report if the 2-in-1 is in tablet or clamshell mode to userspace
by reporting SW_TABLET_MODE events to userspace.

This has been tested on a T100TA, T100CHI, T100HA and T200TA.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>