cpufreq: Make ARM big.LITTLE switcher depend on ARM

The patch currently under review to enable ARM cpufreq drivers for ARM64
which is useful due to the large amount of shared IP between ARM and ARM64
SoCs. However the big.LITTLE switcher relies on an architecture interface
to build which is not present on ARM64. Add a dependency until that is
resolved.

Signed-off-by: Mark Brown <broonie@linaro.org>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>