powerpc/8xx: Don't set IMMR map anymore at boot

Only early debug requires IMMR to be mapped early.

No need to set it up and pin it in assembly. Map it
through page tables at udbg init when necessary.

If CONFIG_PIN_TLB_IMMR is selected, pin it once we
don't need the 32 Mb pinned RAM anymore.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/13c1e8539fdf363d3146f4884e5c3c76c6c308b5.1589866984.git.christophe.leroy@csgroup.eu