powerpc/8xx: fix bogus __init on mmu_mapin_ram_chunk()

Remove __init qualifier for mmu_mapin_ram_chunk() as it is called by
mmu_mark_initmem_nx() and mmu_mark_rodata_ro() which are not __init
functions.

At the same time, mark it static as it is only used in this file.

Reported-by: kbuild test robot <lkp@intel.com>
Fixes: a2227a27

 ("powerpc/32: Don't populate page tables for block mapped pages except on the 8xx")
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/56648921986a6b3e7315b1fbbf4684f21bd2dea8.1576310997.git.christophe.leroy@c-s.fr