ppc64 iSeries: Move setup of systemcfg->platform into iSeries device tree

Add /chosen/linux,platform to the device tree so we can remove iSeries
specific code in setup_system() to set systemcfg->platform.

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