powerpc/eeh: Enhance pcibios_set_pcie_reset_state()

Function pcibios_set_pcie_reset_state() is possibly called by
pci_reset_function(), on which VFIO infrastructure depends to
issue reset. pcibios_set_pcie_reset_state() is issuing reset
on the parent PE of the indicated PCI device. The reset causes
state lost on all PCI devices except the indicated one as the
argument to pcibios_set_pcie_reset_state(). Also, sideband
MMIO access from guest when issuing reset would cause unexpected
EEH error.

For above two issues, the patch applies following enhancements
to pcibios_set_pcie_reset_state():

   * For all PCI devices except the indicated one, save their
     state prior to reset and restore state after that.
   * Explicitly freeze PE prior to reset and unfreeze it after
     that, in order to avoid unexpected EEH error.

Tested-by: Priya M. A <priyama2@in.ibm.com>
Signed-off-by: Gavin Shan <gwshan@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>