85xx: Introduce determine_mp_bootpg() helper.

Match determine_mp_bootpg() that was added for 86xx.  We 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.

Also added a comment about how cpu_reset() is dealing w/an errata on
early 85xx MP HW.

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