sandbox: Add a DSA sandbox driver and unit test

sandbox: Add a DSA sandbox driver and unit test

The DSA sandbox driver is used for unit testing the DSA class code.
It implements a simple 2 port switch plus 1 CPU port, and uses a
very simple tag to identify the ports.

The DSA sandbox device is connected via CPU port to a regular Ethernet
sandbox device, called 'dsa-test-eth, managed by the existing eth
sandbox driver.  The 'dsa-test-eth' is not intended for testing the
eth class code however, but it is used to emulate traffic through the
'lan0' and 'lan1' front pannel switch ports.  To achieve this the dsa
sandbox driver registers a tx handler for the 'dsa-test-eth' device.
The switch ports, labeled as 'lan0' and 'lan1', are also registered
as eth devices by the dsa class code this time.  So pinging through
these switch ports is as easy as:

=> setenv ethact lan0
=> ping 1.2.3.5

Unit tests for the dsa class code were also added.  The 'dsa_probe'
test exercises most API functions from dsa.h.  The 'dsa' unit test
simply exercises ARP/ICMP traffic through the two switch ports,
including tag injection and extraction, with the help of the dsa
sandbox driver.

I took care to minimize the impact on the existing eth unit tests,
though some adjustments needed to be made with the addition of
extra eth interfaces used by the dsa unit tests. The additional eth
interfaces also require MAC addresses, these have been added to the
sandbox default environment.

Signed-off-by: Alex Marginean <alexandru.marginean@nxp.com>
Signed-off-by: Claudiu Manoil <claudiu.manoil@nxp.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Message-Id: <20210216224804.3355044-5-olteanv@gmail.com>
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>