dt-bindings: input: Add common input binding in json-schema

Create schema for the common input properties and merge all properties
from the Documentation/devicetree/bindings/input/keys.txt binding into
this common schema.

Signed-off-by: Michal Vokáč <michal.vokac@ysoft.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>