MIPS: Add accessor macros for 64-bit performance counter registers.

Signed-off-by: David Daney <david.daney@cavium.com>
To: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/2789/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>