MIPS: barrier: Remove fast_mb() Octeon #ifdef'ery

The definition of fast_mb() is the same in both the Octeon & non-Octeon
cases, so remove the duplication & define it only once.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: linux-mips@vger.kernel.org
Cc: Huacai Chen <chenhc@lemote.com>
Cc: Jiaxun Yang <jiaxun.yang@flygoat.com>
Cc: linux-kernel@vger.kernel.org