ARM: at91/dt: sama5d3: Add shutdown controller

The SAMA5D3 has a shutdown controller identical to the sam9x5 SoC family.
Declare it in the DT.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>