dt-bindings: Add OXNAS bindings

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

Documentation/devicetree/bindings/arm/oxnas.txt

+Oxford Semiconductor OXNAS SoCs Family device tree bindings
+-------------------------------------------
+
+Boards with the OX810SE SoC shall have the following properties:
+  Required root node property:
+    compatible: "oxsemi,ox810se"
+
+Board compatible values:
+  - "wd,mbwe" (OX810SE)