ARM: dts: imx6qdl-sabresd.dtsi: Add heartbeat led

Use the red gpio led for heartbeat.

Signed-off-by: Vincent Stehlé <vincent.stehle@freescale.com>
Cc: Shawn Guo <shawn.guo@linaro.org>
Cc: Sascha Hauer <kernel@pengutronix.de>