ARM: dts: Add support for OX820 and Pogoplug V3

Add device tree for the Oxford Seminconductor OX820 SoC and the
Cloud Engines PogoPlug v3 board.
Add the SoC and board compatible strings to oxnas bindings.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>