PCI: thunder-pem: Add support for cn81xx and cn83xx SoCs

The pci-thunder-pem driver was initially developed for cn88xx SoCs.  The
cn81xx and cn83xx members of the same family of SoCs have a slightly
different configuration of interrupt resources in the PEM hardware, which
prevents the INTA legacy interrupt source from functioning with the current
driver.

There are two fixes required:

1) Don't fixup the PME interrupt on the newer SoCs as it already has the
proper value.

2) Report MSI-X Capability Table Size of 2 for the newer SoCs, so the core
MSI-X code doesn't inadvertently clobber the INTA machinery that happens to
reside immediately following the table.

Signed-off-by: David Daney <david.daney@cavium.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>