powerpc/perf: Move perf core & PMU code into a subdirectory

The perf code has grown a lot since it started, and is big enough to
warrant its own subdirectory. For reference it's ~60% bigger than the
oprofile code. It declutters the kernel directory, makes it simpler to
grep for "just perf stuff", and allows us to shorten some filenames.

While we're at it, make it more obvious that we have two implementations
of the core perf logic. One for (roughly) Book3S CPUs, which was the
original implementation, and the other for Freescale embedded CPUs.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>