cpufreq: st: 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: ab0ea257 ("cpufreq: st: Provide runtime initialised driver for ST's platforms")
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>