ar5523: Add new driver

This driver is for the AR5523 chipset from Atheros. It was created
in 2007 by Christoph Hellwig but it was never finished. I found it a
couple of months ago and after some polishing it's working pretty
fine.

The driver was written with the FreeBSD driver (uath) as reference,
which was written with the reverse-engineered windows driver as
reference, hence the feature set is very limited. Station mode
only, no HW crypto offload.

Signed-off-by: Pontus Fuchs <pontus.fuchs@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>