ACPI: thinkpad-acpi: add X61t HKEY events

Tomas Carnecky reports that events 0x5009 and 0x500a are swivel events, and
that 0x500b/0x500c are tablet pen storage bay events.

Document these events, and avoid nasty messages when they happen.

Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Signed-off-by: Len Brown <len.brown@intel.com>