PCI/AER: Define aer_stats structure for AER capable devices

Define a structure to hold the AER statistics.  There are 2 groups of
statistics: dev_* counters that are to be collected for all AER capable
devices and rootport_* counters that are collected for all (AER capable)
rootports only.  Allocate and free this structure when device is added or
released (thus counters survive the lifetime of the device).

Signed-off-by: Rajat Jain <rajatja@google.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>