MIPS: Select CONFIG_ARCH_USE_CMPXCHG_LOCKREF for MIPS64

On MIPS64 we have spinlocks that are 32b in size and an efficient
cmpxchg64 implementation, so we qualify to make use of cmpxchg backed
lockrefs. Select the ARCH_USE_CMPXCHG_LOCKREF Kconfig symbol and provide
a trivial implementation of arch_spin_value_unlocked to satisfy the
lockref code.

Using Linus' simple testcase from
http://article.gmane.org/gmane.linux.file-systems/77466

 on a dual core
system with an in-development MIPS64 CPU running on FPGA I see around an
8% gain:

Pre-patch:
    Total loops: 252698
    Total loops: 251482
    Total loops: 250806
    Total loops: 252885
    Total loops: 251666

Post-patch:
    Total loops: 273728
    Total loops: 269932
    Total loops: 269341
    Total loops: 275004
    Total loops: 270208

[ralf@linux-mips.org: Fixed conflict.]

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: Steven J. Hill <Steven.Hill@imgtec.com>
Cc: linux-kernel@vger.kernel.org
Cc: Maciej W. Rozycki <macro@codesourcery.com>
Cc: Markos Chandras <markos.chandras@imgtec.com>
Patchwork: https://patchwork.linux-mips.org/patch/10810/


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