ACPI: thinkpad-acpi: add CMOS NVRAM polling for hot keys

ACPI: thinkpad-acpi: add CMOS NVRAM polling for hot keys

Older ThinkPad models do not export some of the hot keys over the
event-based ACPI hot key interface.  For these models, one has to poll the
CMOS NVRAM to check the key state at a rate faster than the expected rate
at which the user might repeatedly press the same hot key.

This patch implements this functionality for many of the hotkeys in a
transparent way: hot keys will now Just Work, and the driver knows the best
approach (events or NVRAM polling) to employ, based on the HKEY.MHKA ACPI
method.

Also, the driver can turn off the polling when there are no users for the
hot keys that need such polling.

The NVRAM-based hot keys of the A3x series that have never been implemented
by later models are not supported, to avoid changes in the keymap of the
input devices that could cause headaches in the future.

This patch is based on a previous effort by Richard Hughes.

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