tests/avocado/machine_aspeed.py: Reduce noise on the console for SDK tests

tests/avocado/machine_aspeed.py: Reduce noise on the console for SDK tests

The Aspeed SDK images are based on OpenBMC which starts a lot of
services. The output noise on the console can break from time to time
the test waiting for the logging prompt.

Change the U-Boot bootargs variable to add "quiet" to the kernel
command line and reduce the output volume. This also drops the test on
the CPU id which was nice to have but not essential.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Message-Id: <20221104075347.370503-1-clg@kaod.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20221117172532.538149-3-alex.bennee@linaro.org>