dt-bindings: net: ti,bluetooth: Document default max-speed

Document the default value of max-speed, as used by
linux/drivers/bluetooth/hci_ll.c.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: David Lechner <david@lechnology.com>
Link: https://lore.kernel.org/r/0c6a08c714aeb6dd96b5a54a45b0b5b1cfb49ad1.1635338283.git.geert+renesas@glider.be


Signed-off-by: Rob Herring <robh@kernel.org>