dt-bindings: serial: rs485: remove deprecated .txt binding stub

The plain text binding file was superseded by the YAML schema in
commit d50f974c4f ("dt-bindings: serial: Convert rs485 bindings
to json-schema"). The file now contains only a redirect notice.
Remove it, and update references in serial_core.c and
serial-rs485.rst to point to the YAML schema.

Signed-off-by: Akash Sukhavasi <akash.sukhavasi@gmail.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://patch.msgid.link/20260521162137.6325-1-akash.sukhavasi@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/serial/rs485.txt

@@ -1 +0,0 @@
-See rs485.yaml

Documentation/driver-api/serial/serial-rs485.rst

@@ -132,4 +132,4 @@ RS485 Serial Communications
 6. References
 =============
 
-.. [#DT-bindings]	Documentation/devicetree/bindings/serial/rs485.txt
+.. [#DT-bindings]	Documentation/devicetree/bindings/serial/rs485.yaml

drivers/tty/serial/serial_core.c

@@ -3511,7 +3511,7 @@ EXPORT_SYMBOL_GPL(uart_try_toggle_sysrq);
  * @port: uart device's target port
  *
  * This function implements the device tree binding described in
- * Documentation/devicetree/bindings/serial/rs485.txt.
+ * Documentation/devicetree/bindings/serial/rs485.yaml.
  */
 int uart_get_rs485_mode(struct uart_port *port)
 {