mtd: replace CONFIG_8xx by CONFIG_PPC_8xx

Two config options exist to define powerpc MPC8xx:
* CONFIG_PPC_8xx
* CONFIG_8xx
In addition, CONFIG_PPC_8xx also defines CONFIG_CPM1 as
communication co-processor

arch/powerpc/platforms/Kconfig.cputype has contained the following
comment about CONFIG_8xx item for some years:
"# this is temp to handle compat with arch=ppc"

It looks like not many places still have that old CONFIG_8xx used,
so it is likely to be a good time to get rid of it completely ?

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>