powerpc/8xx: Move additional DTLBMiss handlers out of exception area

When all options are activated, there is not enough space for the
DTLBMiss handlers that handles IMMR area and linear RAM pages in
the exception area once we have added hugepage handling.
So lets move them after .0x2000

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Scott Wood <oss@buserror.net>