powerpc/configs/6xx: Reinstate CONFIG_CPU_FREQ_STAT

In commit 1aefc75b ("cpufreq: stats: Make the stats code
non-modular"), the CPU_FREQ_STAT code was made non-modular. Our
defconfig still said =m though, which meant we no longer got the
code at all. Switch the defconfig to =y.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>