ACPI: ibm-acpi: add header file

Add a (private) header file for ibm-acpi, and move type definitions and
ThinkPad driver constants to the new header file.

This patch has no functional changes.

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