[POWERPC] Fix section mismatch in dart_iommu.c

These functions are only called from __init functions.

WARNING: vmlinux.o(.text+0x398f4): Section mismatch: reference to .init.text:.lmb_alloc (between '.iommu_init_early_dart' and '.pci_dma_bus_setup_dart')

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