ppc64 iSeries: Define /cpus in iSeries device tree

Add the /cpus node and nodes for each cpu, as well as cache size properties,
reg propery, "linux,boot-cpu", and timebase/clock frequency.

With those properties in place we can remove:

- setup_iSeries_cache_sizes()
- code in iSeries_setup_arch() to calculate timebase etc.
- iSeries_calibrate_decr()
- smp_iSeries_numProcs() and simplify smp_iSeries_probe()

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