cpufreq: mediatek: Record previous target vproc value

We found the buck voltage may not be exactly the same with what we set
because CPU may share the same buck with other module.
Therefore, we need to record the previous desired value instead of reading
it from regulators.

Signed-off-by: Andrew-sh.Cheng <andrew-sh.cheng@mediatek.com>
Signed-off-by: Jia-Wei Chang <jia-wei.chang@mediatek.com>
Signed-off-by: Rex-BC Chen <rex-bc.chen@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>