powerpc: Fix cpufreq drivers after cpufreq core changes

This updates the cpufreq drivers in arch/powerpc so they build again
after the core cpufreq changes that broke them in commit
in835481d9bcd65720b473db6b38746a74a3964218.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>