powerpc/86xx: clean up smp init code

Removed the need for asm/mpc86xx.h as it was only used in mpc86xx_smp.c
and just moved the defines it cared about into there.  Also fixed up
the ioremap to only map the one 4k page we need access to and to iounmap
when we are done.

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