ixp4xx/vulcan: add PCI support

Add PCI support for the Vulcan board, supporting USB and CF ports.
The PC/104 bus (actually a hack on the second CarBus slot) is not
currently supported.

Signed-off-by: Marc Zyngier <maz@misterjones.org>
Cc: Imre Kaloz <kaloz@openwrt.org>
Cc: Krzysztof Halasa <khc@pm.waw.pl>
Signed-off-by: Krzysztof Hałasa <khc@pm.waw.pl>