dt-bindings: input: Add document bindings for DA7280

Add device tree binding information for DA7280 haptic driver.
Example bindings for DA7280 are added.

Signed-off-by: Roy Im <roy.im.opensource@diasemi.com>
Reviewed-by: Rob Herring <robh@kernel.org>.
Link: https://lore.kernel.org/r/2bc9a4a9d083ea8f360ec75f6281b6de6c4ef284.1606320459.git.Roy.Im@diasemi.com
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>