MIPS: Define MMIO ordering barriers

Define MMIO ordering barriers as separate operations so as to allow
making places where such a barrier is required distinct from places
where a memory or a DMA barrier is needed.

Architecturally MIPS does not specify ordering requirements for uncached
bus accesses such as MMIO operations normally use and therefore explicit
barriers have to be inserted between MMIO accesses where unspecified
ordering of operations would cause unpredictable results.

MIPS MMIO ordering barriers are implemented using the same underlying
mechanism that memory or a DMA barrier ordering barriers use, that is
either a suitable SYNC instruction or a platform-specific `wbflush'
call.  However platforms may implement different ordering rules for
different kinds of bus activity, so having a separate API makes it
possible to remove unnecessary barriers and avoid a performance hit they
may cause due to unrelated bus activity by making their implementation
expand to nil while keeping the necessary ones.

Also having distinct barriers for each kind of use makes it easier for
the reader to understand what code has been intended to do.

Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Patchwork: https://patchwork.linux-mips.org/patch/20862/
Cc: Ralf Baechle <ralf@linux-mips.org>