powerpc/8xx: Remove immr_map() and immr_unmap()

Since commit fb533d0c ("[POWERPC] 8xx: Infrastructure code cleanup.")
immr_map() is just returning mpc8xxx_immr pointer and immr_unmap()
do nothing.

We already have parts of code that use mpc8xxx_immr directly so get rid
of immr_map() and immr_unmap() by using mpc8xxx_immr directly. And avoid
going through local pointers that hide the pointed structure.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/633ed46f6015ff44d5599258647ea517f75d6a1d.1691474658.git.christophe.leroy@csgroup.eu