cpufreq: mediatek-hw: Separate per-domain and per-instance data

cpufreq: mediatek-hw: Separate per-domain and per-instance data

As it stood, the mediatek cpufreq driver could get away with never
really having a private driver instance struct. This is because all data
was stored in the per-domain structs.

However, this complicates matters when actual per-instance data like the
variant struct is introduced. Instead of having a pointer to it for
every domain, have a pointer to a global "priv" struct that can be
extended over time, and rename the "data" struct to "domain" to
distinguish its purpose better.

Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Nicolas Frattaroli <nicolas.frattaroli@collabora.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>