PCI/PME: Add pcie_walk_rcec() to RCEC PME handling

Root Complex Event Collectors (RCEC) appear as peers of Root Ports and also
have the PME capability. As with AER, there is a need to be able to walk
the RCiEPs associated with their RCEC for purposes of acting upon them with
callbacks.

Add RCEC support through the use of pcie_walk_rcec() to the current PME
service driver and attach the PME service driver to the RCEC device.

Co-developed-by: Qiuxu Zhuo <qiuxu.zhuo@intel.com>
Link: https://lore.kernel.org/r/20201121001036.8560-15-sean.v.kelley@intel.com
Tested-by: Jonathan Cameron <Jonathan.Cameron@huawei.com> # non-native/no RCEC
Signed-off-by: Qiuxu Zhuo <qiuxu.zhuo@intel.com>
Signed-off-by: Sean V Kelley <sean.v.kelley@intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>