cpufreq: mediatek: add support of cpufreq to MT7622 SoC

MT7622 is a 64-bit ARMv8 based dual-core SoC (2 * Cortex-A53) with a
single cluster. The hardware is also compatible with the current driver,
so add MT7622 as one of the compatible string list.

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>