ACPI: thinkpad-acpi: export hotkey maximum masks

The firmware knows how many hot keys it supports, so export this
information in a sysfs attribute.

And the driver knows which keys are always handled by the firmware in all
known ThinkPad models too, so export this information as well in a sysfs
attribute.  Unless you know which events need to be handled in a passive
way, do *not* enable hotkeys that are always handled by the firmware.

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