dt-bindings: Add doc for ROCK Pi 4 A+ and B+

ROCK Pi 4 got 2 more variants called A+ and B+.
Add the dt-bindings documentation for it.
Signed-off-by: Alex Bee <knaerzche@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210618181256.27992-2-knaerzche@gmail.comSigned-off-by: Heiko Stuebner <heiko@sntech.de>

Documentation/devicetree/bindings/arm/rockchip.yaml

@@ -471,11 +471,13 @@ properties:
           - const: radxa,rock
           - const: rockchip,rk3188
 
-      - description: Radxa ROCK Pi 4A/B/C
+      - description: Radxa ROCK Pi 4A/A+/B/B+/C
         items:
           - enum:
               - radxa,rockpi4a
+              - radxa,rockpi4a-plus
               - radxa,rockpi4b
+              - radxa,rockpi4b-plus
               - radxa,rockpi4c
           - const: radxa,rockpi4
           - const: rockchip,rk3399