ppc64 iSeries: Call early_setup() on iSeries

Misc steps to incorporate the flat device tree on iSeries.

- define iseries_probe()
- call build_iSeries_Memory_Map() earlier
- return __pa() of the flat device tree from iSeries_early_setup()
- actually call early_setup() for iSeries
- add iseries_md to machdep_calls
- build prom.o for iSeries
- enable /proc/device-tree for iSeries

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