input: Add event code for accessibility key

HUTRR116 added support for a new usage titled "System Accessibility
Binding" which toggles a system-wide bound accessibility UI or command.
This commit simply adds a new event code for the usage.

Signed-off-by: Aseda Aboagye <aaboagye@chromium.org>
Acked-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Link: https://lore.kernel.org/r/Zl-e97O9nvudco5z@google.com


Signed-off-by: Benjamin Tissoires <bentiss@kernel.org>