powerpc: Fix link errors on 32-bit machines using legacy DMA

The new merged DMA code will try to access isa_bridge_pcidev when
trying to DMA to/from legacy devices. This is however only defined
on 64-bit. Fixes this for now by adding the variable, even if it
stays NULL. In the long run, we'll make isa-bridge.c common to
32 and 64-bit.
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/kernel/pci_32.c

@@ -53,6 +53,12 @@ LIST_HEAD(hose_list);
 
 static int pci_bus_count;
 
+/* This will remain NULL for now, until isa-bridge.c is made common
+ * to both 32-bit and 64-bit.
+ */
+struct pci_dev *isa_bridge_pcidev;
+EXPORT_SYMBOL_GPL(isa_bridge_pcidev);
+
 static void
 fixup_hide_host_resource_fsl(struct pci_dev *dev)
 {