powerpc/powernv: Don't select the cpufreq governors

Deciding wich govenors should be built into the kernel can be left to
users to configure.

Fixes: 81f35902 ("cpufreq: powernv: Select CPUFreq related Kconfig options for powernv")
Signed-off-by: Joel Stanley <joel@jms.id.au>
[mpe: Update powernv/ppc64 defconfigs to enable them by default]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>