linux-yocto: enable powerpc debug fragment

gcc12 generates some warnings with arch/powerpc, and we know that
these warnings are understood/non-fatal at runtime.

This fragment allows testing to continue, while upstream powerpc
looks at removing warnings.

We only enable it for 5.10/5.15 standard kernels for now, since
this gets us enough coverage to continue working with gcc12.

Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 992d9d41)
Signed-off-by: Steve Sakoman <steve@sakoman.com>