[POWERPC] Cleanup CPU inits

Cleanup CPU inits a bit more, Geoff Levand already did some earlier.

* Move CPU state save to cpu_setup, since cpu_setup is only ever done
  on cpu 0 on 64-bit and save is never done more than once.
* Rename __restore_cpu_setup to __restore_cpu_ppc970 and add
  function pointers to the cputable to use instead. Powermac always
  has 970 so no need to check there.
* Rename __970_cpu_preinit to __cpu_preinit_ppc970 and check PVR before
  calling it instead of in it, it's too early to use cputable.
* Rename pSeries_secondary_smp_init to generic_secondary_smp_init since
  everyone but powermac and iSeries use it.

Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Paul Mackerras <paulus@samba.org>