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

Document CalAmp LMU5000.

Signed-off-by: Claudiu Beznea <[e-mail]>
Acked-by: Rob Herring <[e-mail]>
Signed-off-by: Nicolas Ferre <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

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

···
           - const: atmel,sama5d3
           - const: atmel,sama5
 
+      - description: CalAmp LMU5000 board
+        items:
+          - const: calamp,lmu5000
+          - const: atmel,at91sam9g20
+          - const: atmel,at91sam9
+
       - items:
           - enum:
               - atmel,sama5d31