[POWERPC] iseries: Use device tree /system-id in /proc/iSeries/config

We export a bunch of info in /proc/iSeries/config. Currently we pull it
directly out of some iSeries specific structs, but we could use the device
tree instead, this saves decoding it twice and is a little neater.

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