powerpc/cell: Fix dependency in cpufreq

cbe_cpufreq has a partial dependency on cbe_cpufreq_pmi, which cannot
be easily expressed in Kconfig. This fixes it by introducing an
extra Kconfig symbol CBE_CPUFREQ_PMI_ENABLE. To make the dependency
clearer, turn PPC_PMI into an automatic symbol.

Reported-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>