[MIPS] Split up war.h

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