PCI/switchtec: Add Gen4 system info register support

Add the Gen4-specific system info registers and ensure their usage is
guarded by a check on the device's generation.

Link: https://lore.kernel.org/r/20200115035648.2578-6-logang@deltatee.com


Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>