[POWERPC] cell_defconfig: Disable cpufreq and pmi

Cpufreq using pmi is broken by a dependency issue, that
will be fixed in a seperate patch.
Bare-metal cpufreq is broken by hardware limitations.
As it was the only user, pmi is disabled as well.

Signed-off-by: Christian Krafft <krafft@de.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>