cpufreq: mediatek: Add missing MODULE_DEVICE_TABLE

This patch adds missing MODULE_DEVICE_TABLE definition which generates
correct modalias for automatic loading of this cpufreq driver when it is
compiled as an external module.

Signed-off-by: Pali Rohár <pali@kernel.org>
Fixes: 501c574f ("cpufreq: mediatek: Add support of cpufreq to MT2701/MT7623 SoC")
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>