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>