dt-bindings: serial: sc16is7xx: Add description for polling mode

Polling mode is enabled if the "interrupts" property is missing.
Thus, this commit deletes "interrupts" entry from "required" section
and adds a description for the fallback to polling mode at the
"interrupts" entry.

Signed-off-by: Andre Werner <andre.werner@systec-electronic.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20250110073104.1029633-1-andre.werner@systec-electronic.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/serial/nxp,sc16is7xx.yaml

@@ -23,6 +23,8 @@ properties:
     maxItems: 1
 
   interrupts:
+    description:
+      When missing, device driver uses polling instead.
     maxItems: 1
 
   clocks:
@@ -76,7 +78,6 @@ properties:
 required:
   - compatible
   - reg
-  - interrupts
 
 allOf:
   - $ref: /schemas/spi/spi-peripheral-props.yaml#