[PATCH] cell: add platform detection code

I can't really get a conclusive answer from the firmware
people what to check for, so I just try scanning for
anything that starts with "IBM,CPB", which should be
correct for all hardware produced so far and for
systemsim.

Signed-off-by: Arnd Bergmann <arndb@de.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>