cpufreq: mediatek: Add support for MT8176 and MT817x

The Mediatek MT8173 is just one of several SOCs from the same MT817x
family, including the 6-core (4-little/2-big) MT8176.

The mt8173-cpufreq driver supports all of these SOCs, however,
machines using them may use a different machine compatible.

Since this driver checks explicitly for the machine compatible
string, add support for the whole family.

Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>