cpufreq: vexpress-spc: Add missing MODULE_ALIAS

This patch adds missing MODULE_ALIAS for automatic loading of this cpufreq
driver when it is compiled as an external module.

Signed-off-by: Pali Rohár <pali@kernel.org>
Fixes: 47ac9aa1

 ("cpufreq: arm_big_little: add vexpress SPC interface driver")
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>