powerpc/8xx: Fix regression introduced by cache coherency rewrite

After upgrading to the latest kernel on my mpc875 userspace started
running incredibly slow (hours to get to a shell, even!).
I tracked it down to commit 8d30c14c

,
that patch removed a work-around for the 8xx. Adding it
back makes my problem go away.

Signed-off-by: Rex Feany <rfeany@mrv.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>