dt-bindings: serial: Update DT binding docs to support SiFive FU740 SoC

Add new compatible strings to the DT binding documents to support SiFive
FU740-C000.
Signed-off-by: Yash Shah <yash.shah@sifive.com>
Link: https://lore.kernel.org/r/1607403341-57214-5-git-send-email-yash.shah@sifive.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/serial/sifive-serial.yaml

@@ -17,7 +17,9 @@ allOf:
 properties:
   compatible:
     items:
-      - const: sifive,fu540-c000-uart
+      - enum:
+          - sifive,fu540-c000-uart
+          - sifive,fu740-c000-uart
       - const: sifive,uart0
 
     description: