dt-bindings: input: zinitix: Document touch-keys support

In some configurations the touch controller can support the touch-keys.
Document the linux,keycodes property that enables those keys and
specifies the keycodes that should be used to report the key events.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Nikita Travkin <nikita@trvn.ru>
Link: https://lore.kernel.org/r/20240717-zinitix-tkey-v5-1-52ea4cd4bd50@trvn.ru


Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>