MIPS: Correct `mmiowb' barrier for `wbflush' platforms

Redefine `mmiowb' in terms of `iobarrier_w' so that it works correctly
for MIPS I platforms, which have no SYNC machine instruction and use a
call to `wbflush' instead.

This doesn't change the semantics for CONFIG_CPU_CAVIUM_OCTEON, because
`iobarrier_w' expands to `wmb', which is ultimately the same as the
current arrangement.  For MIPS I platforms this not only makes any code
that would happen to use `mmiowb' build and run, but it actually
enforces the ordering required as well, as `iobarrier_w' has it already
covered with the use of `wmb'.

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/20863/
Cc: Ralf Baechle <ralf@linux-mips.org>