dt-bindings: input: adp5588: add bindings

Add device tree bindings for the adp5588-keys driver.

Signed-off-by: Nuno Sá <nuno.sa@analog.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20220829131553.690063-6-nuno.sa@analog.com


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