alx: add a simple AR816x/AR817x device driver

This is a very simple driver, based on the original vendor
driver that Qualcomm/Atheros published/submitted previously,
but reworked to make the code saner. However, it also lost
a number of features (TSO/GSO, VLAN acceleration and multi-
queue support) in the process, as well as debugging support
features I didn't have any use for. The only thing I left
is checksum offload.

More features can obviously be added, but this seemed like
a good start for having a driver in mainline at all.

Johannes Stezenbach has verified that the driver works on
AR8161, I have a AR8171 myself. The E2200 device ID I found
on github in somebody's repository.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: David S. Miller <davem@davemloft.net>