powerpc/44x: Mark mmu_init_secondary() as __init

mmu_init_secondary() calls ppc44x_pin_tlb() which is marked __init,
leading to a warning:

  The function mmu_init_secondary() references
  the function __init ppc44x_pin_tlb().

There's no CPU hotplug support on 44x so mmu_init_secondary() will
only be called at boot. Therefore we should mark it as __init.

Signed-off-by: Alexey Spirkov <alexeis@astrosoft.ru>
[mpe: Flesh out change log details]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>