ARM: dts: at91: sama5d2: add labels to soc dtsi for derivative boards

This adds labels to commonly used device-tree nodes so that derivative
boards can avoid ahb/apb hierarchy.

Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Ludovic Desroches <ludovic.desroches@microchip.com>