[POWERPC] Clean up cpufreq Kconfig dependencies

Shuffle Kconfig order, making the platform drivers menu depend on the global
option instead of each driver being dependent on it.

Also fix dependency of PPC_PMAC on the G5 one.

Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Paul Mackerras <paulus@samba.org>