ARM: dts: at91: shdwc binding: add new shutdown controller documentation

The new shutdown controller compatible with sama5d2 has a new binding
documentation and properties.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sebastian Reichel <sre@kernel.org>