powerpc/powernv: Mark pnv_pci_init_ioda2_phb() as __init

Mark pnv_pci_init_ioda2_phb() as __init.  It is called only from an
init function (pnv_pci_init()), and it calls an init function
(pnv_pci_init_ioda_phb()):

    pnv_pci_init                # init
      pnv_pci_init_ioda2_phb    # non-init
	pnv_pci_init_ioda_phb   # init

This should fix a section mismatch warning.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>