MIPS: Sibyte: Apply M3 workaround only on affected chip types and versions.

Previously it was unconditionally used on all Sibyte family SOCs.  The
M3 bug has to be handled in the TLB exception handler which is extremly
performance sensitive, so this modification is expected to deliver around
2-3% performance improvment.  This is important as required changes to the
M3 workaround will make it more costly.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>