[POWERPC] Fix section mismatch in PCI code

Create a helper function (alloc_maybe_bootmem) that is marked __init_refok
to limit the chances of mistakenly referring to other __init routines.

WARNING: vmlinux.o(.text+0x2a9c4): Section mismatch: reference to .init.text:.__alloc_bootmem (between '.update_dn_pci_info' and '.pci_dn_reconfig_notifier')
WARNING: vmlinux.o(.text+0x36430): Section mismatch: reference to .init.text:.__alloc_bootmem (between '.mpic_msi_init_allocator' and '.find_ht_magic_addr')
WARNING: vmlinux.o(.text+0x5e804): Section mismatch: reference to .init.text:.__alloc_bootmem (between '.celleb_setup_phb' and '.celleb_fake_pci_write_config')
WARNING: vmlinux.o(.text+0x5e8e8): Section mismatch: reference to .init.text:.__alloc_bootmem (between '.celleb_setup_phb' and '.celleb_fake_pci_write_config')
WARNING: vmlinux.o(.text+0x5e968): Section mismatch: reference to .init.text:.__alloc_bootmem (between '.celleb_setup_phb' and '.celleb_fake_pci_write_config')

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>