PCI: thunder: Add driver for ThunderX-pass{1,2} on-chip devices

The cavium,pci-thunder-ecam devices are exactly ECAM-based PCI root
complexes.  These root complexes (loosely referred to as ECAM units in the
hardware manuals) are used to access the Thunder on-chip devices.  They
are special in that all the BARs on devices behind these root complexes are
at fixed addresses.

Add a driver for these devices that synthesizes Enhanced Allocation (EA)
capability entries for each BAR.

Since this EA synthesis is needed for exactly two chip models, we can hard-
code some assumptions about the device topology and the layout of the
config space of specific DEVFNs in the driver.

[bhelgaas: changelog, whitespace]
Signed-off-by: David Daney <david.daney@cavium.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Rob Herring <robh@kernel.org>