Bluetooth: Introduce BT_HCIUART_RTL configuration option

Like all the other UART protocols, introduce a configuration option for
Realtek based serial devices.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>