powerpc/powernv/pci: Use kzalloc() for phb related allocations

As part of commit fbbefb32 ("powerpc/pci: Move PHB discovery for
PCI_DN using platforms"), I switched some allocations from
memblock_alloc() to kmalloc(), otherwise memblock would warn that it
was being called after slab init.

However I missed that the code relied on the allocations being zeroed,
without which we could end up crashing:

  pci_bus 0000:00: busn_res: [bus 00-ff] end is updated to ff
  BUG: Unable to handle kernel data access on read at 0x6b6b6b6b6b6b6af7
  Faulting instruction address: 0xc0000000000dbc90
  Oops: Kernel access of bad area, sig: 11 [#1]
  LE PAGE_SIZE=64K MMU=Hash SMP NR_CPUS=2048 NUMA PowerNV
  ...
  NIP  pnv_ioda_get_pe_state+0xe0/0x1d0
  LR   pnv_ioda_get_pe_state+0xb4/0x1d0
  Call Trace:
    pnv_ioda_get_pe_state+0xb4/0x1d0 (unreliable)
    pnv_pci_config_check_eeh.isra.9+0x78/0x270
    pnv_pci_read_config+0xf8/0x160
    pci_bus_read_config_dword+0xa4/0x120
    pci_bus_generic_read_dev_vendor_id+0x54/0x270
    pci_scan_single_device+0xb8/0x140
    pci_scan_slot+0x80/0x1b0
    pci_scan_child_bus_extend+0x94/0x490
    pcibios_scan_phb+0x1f8/0x3c0
    pcibios_init+0x8c/0x12c
    do_one_initcall+0x94/0x510
    kernel_init_freeable+0x35c/0x3fc
    kernel_init+0x2c/0x168
    ret_from_kernel_thread+0x5c/0x70

Switch them to kzalloc().

Fixes: fbbefb32

 ("powerpc/pci: Move PHB discovery for PCI_DN using platforms")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210211112749.3410771-1-mpe@ellerman.id.au