ARM: at91: dt: add device tree file for SAMA5D4ek board

Add reference SAMA5D4-EK platform DT file.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Josh Wu <josh.wu@atmel.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>