thinkpad-acpi: add internal hotkey event API

Add an internal API to the driver, to allow subdrivers to request and
receive HKEY 0x1000 events.  This API will be used by the backlight
(brightness up/down) and upcoming ALSA mixer (volume up/down/mute)
subdrivers.

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