CNS3xxx: Fix logical PCIe topology.

Without this patch, each root port and the device connected directly to it seem
to be located on a shared (virtual) bus #0. It creates problems with enabling
devices (the PCI code doesn't know that the root bridge must be enabled in order
to access other devices).
The PCIe topology shown by lspci doesn't reflect reality, e.g.:

0000:00:00.0 PCI bridge: Cavium Networks Device 3400
0000:00:01.0 PCI bridge: Texas Instruments XIO2001 PCI Express-to-PCI Bridge
0000:02:...
0001:00:00.0 PCI bridge: Cavium Networks Device 3400 (for the second lane/bus)

-+-[0001:00]---00.0-[01]--
 \-[0000:00]-+-00.0-[01]--
             | ^^^^ root bridge
             \-01.0-[02]----...
               ^^^^ first external device

With this patch, the first external PCIe device is connected to bus #1
(behind the root bridge).

-+-[0001:00]---00.0-[01]--
 \-[0000:00]---00.0-[01-02]----------00.0-[02]----...
               ^^^^ root bridge      ^^^^ first external device

Signed-off-by: Krzysztof Hałasa <khalasa@piap.pl>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>