thinkpad-acpi - Move thinkpad-acpi.txt to Documentation/laptops

Also update references to thinkpad-acpi.txt in Kconfig.

Signed-off-by: Carlos Corbacho <carlos@strangeworlds.co.uk>
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
CC: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Signed-off-by: Len Brown <len.brown@intel.com>