cpufreq: mediatek-hw: Add support for CPUFREQ HW

Introduce cpufreq HW driver which can support
CPU frequency adjust in MT6779 platform.

Signed-off-by: Hector.Yuan <hector.yuan@mediatek.com>
[ Viresh: Massaged the patch and cleaned some stuff. ]
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>