dt-bindings: serial: add RX and TX FIFO properties

Add two optional DT properties to configure RX and TX FIFO thresholds:
- rx-threshold
- tx-threshold

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Erwan Le Ray <erwan.leray@foss.st.com>
Link: https://lore.kernel.org/r/20210413174015.23011-2-erwan.leray@foss.st.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/serial/serial.yaml

@@ -75,6 +75,16 @@ properties:
     type: boolean
     description: CTS and RTS pins are swapped.
 
+  rx-threshold:
+    $ref: /schemas/types.yaml#/definitions/uint32
+    description:
+      RX FIFO threshold configuration (in bytes).
+
+  tx-threshold:
+    $ref: /schemas/types.yaml#/definitions/uint32
+    description:
+      TX FIFO threshold configuration (in bytes).
+
 if:
   required:
     - uart-has-rtscts