ARM: at91/dt: sama5d4 xplained: enable the led D8

Enable D8 led as now the pioD is available. D8 is the power led and is on by
default.

Signed-off-by: Josh Wu <josh.wu@atmel.com>
[nicolas.ferre@atmel.com: split the patch in 2]
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>