ARM: dts: at91: sama5d2 Xplained: add leds node

Add the three leds on the sama5d2 Xplained board with their pinctrl node.
The blue led is positioned with the "heartbeat" trigger.

Signed-off-by: Wenyou Yang <wenyou.yang@atmel.com>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
[nicolas.ferre@atmel.com: add commit message and adapt to newer kernel]
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>