powerpc/8xx: use the fixmapped IMMR in cpm_reset()

Since commit f86ef74e ("powerpc/8xx: Fix vaddr for IMMR early
remap"), the IMMR area has been mapped at startup with fixmap.

Use that fixmap directly instead of calling ioremap(), this
avoids calling ioremap() early before the slab is available.

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/f816ccdbd15b97cf43c5a8c7cc8dfa8db58ff036.1568294935.git.christophe.leroy@c-s.fr