Enable devfreq events along with drivers for scaling frequency and
voltages of Exynos buses and Dynamic Memory Controller (DMC).  This
usually brings energy saving benefits.

So far devfreq was disabled because it was causing hangs during system
reboot (voltage not matching reset frequency). This was already fixed.

Occasionally, devfreq might negatively impact performance of certain SoC
blocks, e.g. when a bus is scaled down but some block (like Mixer with
two Full HD windows) wants to perform high-throughput DMA operations.
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>