Bluetooth: Support for Atheros AR300x serial chip

Implements Atheros AR300x serial HCI protocol.

This protocol extends H4 serial protocol to implement enhanced power
management features supported by Atheros AR300x serial Bluetooth chipsets.

Signed-off-by: Suraj Sumangala <suraj@atheros.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>