powerpc/8xx: Allow pinning IMMR TLB when using early debug console

CONFIG_EARLY_DEBUG_CPM requires IMMR area TLB to be pinned
otherwise it doesn't survive MMU_init, and the boot fails.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>