[MIPS] 20K: Handle WAIT related bugs according to errata information

We used to avoid the WAIT entirely on the 20K but really only need to do
this on early revs of the 20K.  Without this a 20K was a bit of a
power hog.  Well, in the lower power power hog category ;-)

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