powerpc/cpm2: Remove cpm2_map() and cpm2_unmap()

Since commit 449012da ("[POWERPC] cpm2: Infrastructure code
cleanup.") cpm2_map() is just returning cpm2_immr pointer and
cpm2_unmap() does nothing.

We already have parts of code that use cpm2_immr directly so get rid
of cpm2_map() and cpm2_unmap() by using cpm2_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/9fe6ff7284e9f968b12abe7de7c08d7ea40e29d6.1691474658.git.christophe.leroy@csgroup.eu