Add MIPS support

Add MIPS support

[ Edit by Mathieu Desnoyers: add explanations about supported
MIPS architectures, extracted from conversation with Ralf Baechle:

* Supported architectures

Ralf Baechle (edited by Mathieu Desnoyers):

This code works on all MIPS architecture variants.  The memory barrier
instruction, SYNC, was introduced for MIPS II.  The original MIPS I
instruction set doesn't have SYNC nor SMP support in the processor
architecture itself so SMP required horrible kludges in the system
hardware.  I think it's safe to say that Linux/MIPS will never support
any of these MIPS I SMP systems.  In the unlikely case this happens
anyway, we have a (Linux) kernel emulation of the SYNC instruction.
Voila - full binary compatibility across all MIPS processors and the
oldest hardware pays the performance penalty.

* Choice of barrier for cmm_mb()/cmm_rmb()/cmm_wmb()

Ralf Baechle:
"RMI (aka Netlogic and now Broadcom) XLR processor cores can be
configured to permit LD-LD, LD-ST, ST-LD and ST-ST reordering; default
is only ST-ST reordering.  To allow Linux to eventually enable full
reordering cmm_mb(), cmm_rmb() and cmm_wmb() all should perform SYNC
and a compiler barrier."

* No-op choice for cmm_read_barrier_depends():

Ralf Baechle:
"Technically there is nothing in the MIPS architecture spec that would
keep a MIPS implementation from reordering as freely as an Alpha or
even more liberally.  In practice most do strong ordering.  However
there is no MIPS implementation that makes full use of all the rope
provided.  So in theory a paranoid implementation of
cmm_read_barrier_depends() for MIPS should perform a SYNC.  In reality
it's not necessary and no sane MIPS core designer would implement
something that would design a core that need a non-empty
cmm_read_barrier_depends().  The reason why my patch had an empty one
is that I was using the Alpha code as a template."

Mathieu Desnoyers:
Moreover, the Linux kernel chooses a no-op for MIPS
read_barrier_depends() implementation, so any MIPS architecture that
would be as weak as Alpha would break the Linux kernel before breaking
the userspace RCU library.

* No need to put ".set noreorder" in cmm_mb() inline assembly:

Ralf Baechle:
"Certain instructions such as SYNC won't get reordered." ]

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
CC: Paul McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>