Documentation: ABI: Add ABI file for legacy /proc/i8k interface

Add ABI file for informing remaining users of the
deprecation of the legacy /proc/i8k interface.

Signed-off-by: Armin Wolf <W_Armin@gmx.de>
Link: https://lore.kernel.org/r/20220109214248.61759-4-W_Armin@gmx.de
Signed-off-by: Guenter Roeck <linux@roeck-us.net>