ARM: dts: at91: sama5d2: add AIC node

When using interrupts property, a global interrupt controller needs to
be added to avoid warnings when compiling device-tree:

 arch/arm/dts/at91-sama5d2_xplained.dtb: Warning (interrupts_property):
    /ahb/apb/timer@f800c000: Missing interrupt-parent

Add AIC node as the sama5d2 global interrupt controller.

Signed-off-by: Clément Léger <clement.leger@bootlin.com>