ACPI: thinkpad-acpi: make the input event mode the default

Make the input layer the default way to deal with thinkpad-acpi hot keys,
but add a kernel config option to retain the old way of doing things.

This means we map a lot more keys to useful stuff by default, and also that
we enable hot key handling by default on driver load (like Windows does).

The documentation for proper use of this resource is also updated.

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