dt-bindings: arm: rockchip: Add Rock Pi N8 binding

Rock Pi N8 is a Rockchip RK3288 based SBC, which has
- VMARC RK3288 SOM (as per SMARC standard) from Vamrs.
- Compatible carrier board from Radxa.

VMARC RK3288 SOM need to mount on top of dalang carrier
board for making Rock PI N8 SBC.

Add dt-bindings for it.
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200715083418.112003-6-jagan@amarulasolutions.comSigned-off-by: Heiko Stuebner <heiko@sntech.de>

Documentation/devicetree/bindings/arm/rockchip.yaml

@@ -435,6 +435,12 @@ properties:
           - const: radxa,rockpi4
           - const: rockchip,rk3399
 
+      - description: Radxa ROCK Pi N8
+        items:
+          - const: radxa,rockpi-n8
+          - const: vamrs,rk3288-vmarc-som
+          - const: rockchip,rk3288
+
       - description: Radxa ROCK Pi N10
         items:
           - const: radxa,rockpi-n10