input: cros_ec_keyb: Add non-matrix buttons and switches

On some newer boards using mkbp we're hooking up non-matrix buttons and
switches to the EC but NOT to the main application processor.

Let's add kernel support to handle this.  Rather than creating a whole
new input driver, we'll continue to use cros_ec_keyb and just report the
new keys.
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Acked-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>