dt-bindings: input: Centralize 'linux,code' definition

Multiple bindings use 'linux,code', but there is not a central
definition and type. Add 'linux,code' to input.yaml and update all the
users to use it.

Signed-off-by: Rob Herring <robh@kernel.org>
Acked-by: Samuel Holland <samuel@sholland.org>
Acked-by: Jeff LaBundy <jeff@labundy.com>
Acked-by: Artur Rojek <contact@artur-rojek.eu>
Link: https://lore.kernel.org/r/20220608211207.2058487-3-robh@kernel.org


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