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 <[e-mail]>
Cc: "David S. Miller" <[e-mail]>
Cc: Jakub Kicinski <[e-mail]>
Cc: [e-mail]
Cc: Vasily Khoruzhick <[e-mail]>
Reviewed-by: Alistair Francis <[e-mail]>
Reviewed-by: Rob Herring <[e-mail]>
Signed-off-by: Maxime Ripard <[e-mail]>
Signed-off-by: Rob Herring <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

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

···
     maxItems: 1
     description: GPIO specifier, used to wakeup the host processor
 
+  max-speed: true
+
 required:
   - compatible