greybus: svc: add key event handling

Add a new input device associated with the SVC to handle key events.
This new events are transfer over a new greybus svc operation which is
unidirectional.

It was selected the KEY_A for representing the KEY_ARA_BUTTON key code.

Signed-off-by: Rui Miguel Silva <rui.silva@linaro.org>
Reviewed-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>