[ARM] 3438/1: ixp23xx: add pci slave support

Patch from Lennert Buytenhek

On the Double Espresso board, the IXP2350s are PCI slave devices and
we skip calling pci_common_init() as that enumerates the bus.  But even
though we are a PCI slave device, there is still some PCI-related setup
that has to be done.

Create ixp23xx_pci_common_init(), move the common initialisation bits
there, and have this function called from both the PCI master and the
PCI slave init path.

Signed-off-by: Lennert Buytenhek <buytenh@wantstofly.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>