[POWERPC] 8xx: Add mpc885ads support and common mpc8xx files

This adds the core 8xx stuff and specifically mpc885ads board-specific
bits to arch/powerpc. Respective Kconfig has been cleaned up from the stuff
not yet ported over to avoid confusion. Updated and cleaned version.

Signed-off-by: Vitaly Bordug <vbordug@ru.mvista.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>