Restore the smt-enabled=off kernel command line functionality:

- Remove the SMT_DYNAMIC state now that smt_snooze_delay allows for the
  same thing.
- Remove the early prom.c parsing for the option, put it into an
  early_param instead.
- In setup_cpu_maps honour the smt-enabled setting

Note to Nathan: In order to allow cpu hotplug add of secondary threads after
booting with smt-enabled=off, I had to initialise cpu_present_map to
cpu_online_map in smp_cpus_done.  Im not sure how you want to handle this but
it seems our present map currently does not allow cpus to be added into the
partition that werent there at boot (but were in the possible map).
Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>