powerpc: Fix crash on CPU hotplug

early_init_mmu_secondary() is called at CPU hotplug time, so it
must be marked as __cpuinit, not __init.

Caused by 757c74d2

 ("powerpc/mm: Introduce early_init_mmu() on 64-bit").

Tested-by: Sachin Sant <sachinp@in.ibm.com>
Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>