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>

arch/powerpc/configs/ppc6xx_defconfig

@@ -61,7 +61,7 @@ CONFIG_SBC8641D=y
 CONFIG_MPC8610_HPCD=y
 CONFIG_GEF_SBC610=y
 CONFIG_CPU_FREQ=y
-CONFIG_CPU_FREQ_STAT=m
+CONFIG_CPU_FREQ_STAT=y
 CONFIG_CPU_FREQ_DEFAULT_GOV_USERSPACE=y
 CONFIG_CPU_FREQ_GOV_PERFORMANCE=y
 CONFIG_CPU_FREQ_GOV_POWERSAVE=m