ppc4xx: Add routine to retrieve CPU number

Provide a weak defined routine to retrieve the CPU number for
reference boards that have multiple CPU's.  Default behavior
is the existing single CPU print output.  Reference boards with
multiple CPU's need to provide a board specific routine.
See board/amcc/arches/arches.c for an example.

Signed-off-by: Adam Graham <agraham@amcc.com>
Signed-off-by: Victor Gallardo <vgallardo@amcc.com>
Signed-off-by: Stefan Roese <sr@denx.de>