[PATCH] ppc32: Allow user to individual select CHRP/PMAC/PREP config

Made the CHRP/PMAC/PREP config options selectable by the user.
This allows us to build kernels specifically for one of the
platforms thus reducing code size.

Signed-off-by: Kumar Gala <kumar.gala@freescale.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>