selftests/powerpc: Suggest memtrace instead of /dev/mem for ci memory

The suggested alternative for getting cache-inhibited memory with 'mem='
and /dev/mem is pretty hacky. Also, PAPR guests do not allow system
memory to be mapped cache-inhibited so despite /dev/mem being available
this will not work which can cause confusion.  Instead recommend using
the memtrace buffers. memtrace is only available on powernv so there
will not be any chance of trying to do this in a guest.

Signed-off-by: Jordan Niethe <jniethe5@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210225032108.1458352-2-jniethe5@gmail.com