powerpc/xmon: Dump ftrace buffers for the current CPU only

Current xmon 'dt' command dumps the tracing buffer for all the CPUs,
which makes it very hard to read due to the fact that most of
powerpc machines currently have many CPUs. Other than that, the CPU
lines are interleaved in the ftrace log.

This new option just dumps the ftrace buffer for the current CPU.

Signed-off-by: Breno Leitao <leitao@debian.org>
Acked-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>