dt-bindings: ARM: at91: document CalAmp LMU5000 board

Document CalAmp LMU5000.
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Link: https://lore.kernel.org/r/20210816064416.1630674-4-claudiu.beznea@microchip.com

Documentation/devicetree/bindings/arm/atmel-at91.yaml

@@ -126,6 +126,12 @@ properties:
           - const: atmel,sama5d3
           - const: atmel,sama5
 
+      - description: CalAmp LMU5000 board
+        items:
+          - const: calamp,lmu5000
+          - const: atmel,at91sam9g20
+          - const: atmel,at91sam9
+
       - items:
           - enum:
               - atmel,sama5d31