ppc64 iSeries: Create a fake flat device tree on iSeries

This patch adds infrastructure for creating a fake flattened device tree
on iSeries.

We also need to build prom.o for iSeries which means we'll always need it.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>