dt-bindings: input: Add the PinePhone keyboard binding

Add devicetree support for the PinePhone keyboard case, which provides a
matrix keyboard interface and a proxied I2C bus.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Samuel Holland <samuel@sholland.org>
Link: https://lore.kernel.org/r/20220618165747.55709-2-samuel@sholland.org
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>