samples: input_dump: add a board overlay for testing zephyr,gpio-keys

Add an initial board overlay in the input_dump directory for testing
zephyr,gpio-keys. This takes the exsiting "gpio-keys" node, changes the
compatible to the one used by the gpio keys input driver and set some
codes.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>