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: NRalf Baechle <ralf@linux-mips.org>

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

It was getting a little big, ugly and a primary source for merge conflicts.
Also the old method was a bit too forgiving in that the workaround did
default to off, so now there is an explicit #error forcing platform
maintainers to think if they should enable a workaround for a particular
platform.
Signed-off-by: NRalf Baechle <ralf@linux-mips.org>