archs38: switch to 5.15 by default

archs38: switch to 5.15 by default

It appears that only a few users are using the archs38 SoC. The most
active user of the target has already approved the testing kernel and
so it is very unlikely bugs will be reported in the near future.
Therefore, the target should be directly bumped to 5.15.

Signed-off-by: Nick Hainke <vincent@systemli.org>