dt-bindings: Add doc for Xunlong OrangePi R1 Plus LTS

Add devicetree binding documentation for the
Xunlong OrangePi R1 Plus LTS.

Signed-off-by: Tianling Shen <cnsztl@gmail.com>
Acked-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20230325074022.9818-4-cnsztl@gmail.com
Signed-off-by: Heiko Stuebner <heiko@sntech.de>

Author: 1715173329

Documentation/devicetree/bindings/arm/rockchip.yaml

@@ -831,9 +831,11 @@ properties:
           - const: tronsmart,orion-r68-meta
           - const: rockchip,rk3368
 
-      - description: Xunlong Orange Pi R1 Plus
+      - description: Xunlong Orange Pi R1 Plus / LTS
         items:
-          - const: xunlong,orangepi-r1-plus
+          - enum:
+              - xunlong,orangepi-r1-plus
+              - xunlong,orangepi-r1-plus-lts
           - const: rockchip,rk3328
 
       - description: Zkmagic A95X Z2