MIPS: perf: Add support for 64-bit perf counters.

The hard coded constants are moved to struct mips_pmu.  All counter
register access move to the read_counter and write_counter function
pointers, which are set to either 32-bit or 64-bit access methods at
initialization time.

Many of the function pointers in struct mips_pmu were not needed as
there was only a single implementation, these were removed.

I couldn't figure out what made struct cpu_hw_events.msbs[] at all
useful, so I removed it too.

Some functions and other declarations were reordered to reduce the
need for forward declarations.
Signed-off-by: David Daney <david.daney@cavium.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Deng-Cheng Zhu <dengcheng.zhu@gmail.com>
To: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/2792/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>