MIPS: Remove execution hazard barriers for Octeon.

The Octeon has no execution hazards, so we can remove them and save an
instruction per TLB handler invocation.

Signed-off-by: David Daney <ddaney@caviumnetworks.com>
Reviewed by: David VomLehn <dvomlehn@cisco.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>