powerpc/msi: Fix section mismatch warning in msi_bitmap_alloc()

Building with CONFIG_DEBUG_SECTION_MISMATCH gives the following warning:

  The function .msi_bitmap_alloc() references
  the function __init .memblock_virt_alloc_try_nid().

Memory allocation in msi_bitmap_alloc() uses either slab allocator or
memblock boot time allocator depending on slab_is_available().

So the section mismatch warning is correct, but in practice there is no
bug so mark msi_bitmap_alloc() as __init_refok.

Signed-off-by: Denis Kirjanov <kda@linux-powerpc.org>
[mpe: Flesh out change log a bit]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>