This patch adds support for the atusb transceiver.

The current driver supports basic functionality only. Possible further
tasks would be to sync functionality with the at86rf230 driver, because
the atusb use internally an at86rf231 transceiver. Some of these
features need a firmware update like AACK and ARET handling.

I did small changes to this driver to work with xmit_async callback and
setting of a random extended perm address.
Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Cc: Werner Almesberger <werner@almesberger.net>
Cc: Stefan Schmidt <s.schmidt@samsung.com>
Cc: Richard Sharpe <realrichardsharpe@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>