powerpc/powernv/eeh: Fix oops when probing cxl devices

Recent cleanup in the way EEH support is added to a device causes a
kernel oops when the cxl driver probes a device and creates virtual
devices discovered on the FPGA:

  BUG: Kernel NULL pointer dereference at 0x000000a0
  Faulting instruction address: 0xc000000000048070
  Oops: Kernel access of bad area, sig: 7 [#1]
  ...
  NIP eeh_add_device_late.part.9+0x50/0x1e0
  LR  eeh_add_device_late.part.9+0x3c/0x1e0
  Call Trace:
    _dev_info+0x5c/0x6c (unreliable)
    pnv_pcibios_bus_add_device+0x60/0xb0
    pcibios_bus_add_device+0x40/0x60
    pci_bus_add_device+0x30/0x100
    pci_bus_add_devices+0x64/0xd0
    cxl_pci_vphb_add+0xe0/0x130 [cxl]
    cxl_probe+0x504/0x5b0 [cxl]
    local_pci_probe+0x6c/0x110
    work_for_cpu_fn+0x38/0x60

The root cause is that those cxl virtual devices don't have a
representation in the device tree and therefore no associated pci_dn
structure. In eeh_add_device_late(), pdn is NULL, so edev is NULL and
we oops.

We never had explicit support for EEH for those virtual devices.
Instead, EEH events are reported to the (real) pci device and handled
by the cxl driver. Which can then forward to the virtual devices and
handle dependencies. The fact that we try adding EEH support for the
virtual devices is new and a side-effect of the recent cleanup.

This patch fixes it by skipping adding EEH support on powernv for
devices which don't have a pci_dn structure.

The cxl driver doesn't create virtual devices on pseries so this patch
doesn't fix it there intentionally.

Fixes: b905f8cd ("powerpc/eeh: EEH for pSeries hot plug")
Signed-off-by: Frederic Barrat <fbarrat@linux.ibm.com>
Reviewed-by: Sam Bobroff <sbobroff@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20191016162833.22509-1-fbarrat@linux.ibm.com