devfreq: Remove MODULE_ALIAS for exynos4 busfreq driver

This driver can only be built-in, it does not make sense to add modalias for
it (in addition to being incorrect, the platform modalias needs to be prefixed
with "platform:").

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>