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

Make the red gpio led available to the user.

This can be toggled with the sysfs for example, or used as a heartbeat or mmc
activity light by changing the trigger.

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