arm64: Add big.LITTLE switcher stub

arm64: Add big.LITTLE switcher stub

The big.LITTLE cpufreq driver is useful on arm64 big.LITTLE systems even
without IKS support since it implements support for clusters with shared
clocks (a common big.LITTLE configuration). In order to allow it to be
built provide the non-IKS stubs for arm64, enabling cpufreq with all the
cores available.

It may make sense to make an asm-generic version of these stubs instead but
given that there's only likely to be these two architectures using the code
and asm-generic stubs also need per architecture updates it's probably more
trouble than it's worth.

Signed-off-by: Mark Brown <broonie@linaro.org>