dt-bindings: riscv: Update YAML doc to support SiFive HiFive Unmatched board

Add new compatible strings to the YAML DT binding document to support
SiFive's HiFive Unmatched board
Signed-off-by: Yash Shah <yash.shah@sifive.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>

Documentation/devicetree/bindings/riscv/sifive.yaml

@@ -17,12 +17,19 @@ properties:
   $nodename:
     const: '/'
   compatible:
-    items:
+    oneOf:
+      - items:
           - enum:
               - sifive,hifive-unleashed-a00
           - const: sifive,fu540-c000
           - const: sifive,fu540
 
+      - items:
+          - enum:
+              - sifive,hifive-unmatched-a00
+          - const: sifive,fu740-c000
+          - const: sifive,fu740
+
 additionalProperties: true
 
 ...