ARM: dts: cros-ec-keyboard: Add LOCK key to keyboard matrix

The LOCK key is at KSO9/KSI3 for Chromebook Flip and other devices
that use the Chrome OS EC keyboard matrix.

Signed-off-by: James Chao <james_chao@asus.com>
Signed-off-by: YH Huang <yh.huang@mediatek.com>
Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>