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