[POWERPC] Split out CPU specific options into a new Kconfig file

A lot of the options in arch/powerpc/Kconfig deal with the CPU menu,
and my next patches add more to them.  Moving them to a new
arch/powerpc/platforms/Kconfig.cputype file makes it easier to
follow.

There are no functional changes in here.

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