cpufreq: arm_big_little: add cluster regulator support

cpufreq: arm_big_little: add cluster regulator support

Add cluster regulator support as a preparation to adding
generic arm_big_little_dt cpufreq_dt driver support for
ODROID-XU3 board.  This allows arm_big_little[_dt] driver
to set not only the frequency but also the voltage (which
is obtained from operating point's voltage value) for CPU
clusters.

Cc: Kukjin Kim <kgene.kim@samsung.com>
Cc: Doug Anderson <dianders@chromium.org>
Cc: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Cc: Andreas Faerber <afaerber@suse.de>
Cc: Sachin Kamat <sachin.kamat@linaro.org>
Cc: Thomas Abraham <thomas.ab@samsung.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>