powerpc/pseries: PCIE PHB reset

Several device drivers hit EEH(Extended Error handling) when
triggering kdump on Pseries PowerVM. This patch implemented a reset of
the PHBs in pci general code when triggering kdump. PHB reset stop all
PCI transactions from normal kernel. We have tested the patch in
several enviroments:
  - direct slot adapters
  - adapters under the switch
  - a VF adapter in PowerVM
  - a VF adapter/adapter in KVM guest.

Signed-off-by: Wen Xiong <wenxiong@linux.vnet.ibm.com>
[mpe: Fix broken whitespace, subject & SOB formatting]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/1594651173-32166-1-git-send-email-wenxiong@linux.vnet.ibm.com