PCI: iproc: Improve core register population

As the number of iProc PCIe core registers starts to grow and differ
between different revisions of the iProc PCIe controllers, the
current way of populating each individual unsupported register with
value 'IPROC_PCIE_REG_INVALID' with a table entry has become a bit
messy and is difficult to scale up in the future.

Improve the current driver by populating the invalid entries with code
instead of through individual table entries.  This helps to avoid a
significant number of invalid table entries when support for the next
revision of the iProc controller is added.

Signed-off-by: Ray Jui <ray.jui@broadcom.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Scott Branden <scott.branden@broadcom.com>