cpufreq: sti: add missing MODULE_DEVICE_TABLE entry for stih418

'st,stih418' is missing in the compatible list.
Add it in order to use the driver with stih418 platform.

Signed-off-by: Raphael Gallais-Pou <rgallaispou@gmail.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>