exynos: Voltage control interfaces

Based on gokhanmoral's combo patch at:
https://github.com/gokhanmoral/siyahkernel3/commit/c9d9ad92c7c97c7df7b55968b04b34c50ef3ed9e

I prefer to do things in smaller chunks - voltage control first,
clock control later.

Signed-off-by: Andrew Dodd <atd7@cornell.edu>
exynos-cpufreq: Ensure that scaling_available_frequencies is populated

Based on patches by netarchy

Signed-off-by: Andrew Dodd <atd7@cornell.edu>