powerpc/8xx: Fix permanently mapped IMMR region.

When not using large TLBs, the IMMR region is still
mapped as a whole block in the FIXMAP area.

Properly report that the IMMR region is block-mapped even
when not using large TLBs.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/45f4f414bcd7198b0755cf4287ff216fbfc24b9d.1574774187.git.christophe.leroy@c-s.fr