ARM: dts: cubox: Map gpio-keys to gpio3 8

The Cubox has a recessed button between the HDMI and RJ-45 connectors
that wasn't mapped in the device tree, so I've mapped it to gpio-keys
BTN_0.

Signed-off-by: George Joseph <george.joseph@fairview5.com>
Tested-by: George Joseph <george.joseph@fairview5.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>