86xx: Cleanup MP support

* Use CONFIG_MP instead of CONFIG_NUM_CPUS to match 85xx
* Introduce determine_mp_bootpg() helper.  We'll need this to address a
  bug introduced in v2009.03 with 86xx MP booting.  We have to make sure
  to reserve the region of memory used for the MP bootpg() so other
  u-boot code doesn't use it.
* Added dummy versions of cpu_reset(), cpu_status() & cpu_release() to
  allow cmd_mp.c to build and work. In the future we should look at
  implementing all these functions. This could be common w/85xx if we
  use spin tables on 86xx.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>