ARM: dts: i.MX51 babbage: Support diagnostic LED

The D25 LED controlled by gpio on the i.MX51 babbage
board is a diagnostic LED according to the board design.
This patch adds the relevant device tree nodes to the
i.MX51 babbage device tree file to support this LED.

Signed-off-by: Liu Ying <Ying.Liu@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>