ACPI: thinkpad-acpi: checkpoint sysfs interface version due to hotkey

The change in the size of the hotkey mask, the hability to report the keys
that use the higher bits, and the addition of the hotkey_radio_sw attribute
are important enough features to warrant increasing the minor field of the
sysfs interface version.

Also, document a bit better how and when the thinkpad-acpi sysfs interface
version will be updated.

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