dt-bindings: bluetooth: realtek: Add missing max-speed

additionalProperties prevent any property not explicitly defined in the
binding to be used. Yet, some serial properties like max-speed are valid
and validated through the serial/serial.yaml binding.

Even though the ideal solution would be to use unevaluatedProperties
instead, it's not pratical due to the way the bus bindings have been
described. Let's add max-speed to remove the warning.

Cc: Alistair Francis <alistair@alistair23.me>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: netdev@vger.kernel.org
Cc: Vasily Khoruzhick <anarsoul@gmail.com>
Reviewed-by: Alistair Francis <alistair@alistair23.me>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210924072756.869731-2-maxime@cerno.tech

Documentation/devicetree/bindings/net/realtek-bluetooth.yaml

@@ -34,6 +34,8 @@ properties:
     maxItems: 1
     description: GPIO specifier, used to wakeup the host processor
 
+  max-speed: true
+
 required:
   - compatible