MIPS: perf: add I6500 handling

Add a definition of the perf registers for the new I6500 core.

Since I6500 has the same event definitions as I6400, re-use the existing
i6400 map structures by renaming them to a slightly more generic
'i6x00_***_map'.

Signed-off-by: Marcin Nowakowski <marcin.nowakowski@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/16362/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>