devfreq: Fix build break of devfreq-event class

This patch fixes the build break of the exynos-ppmu driver because Makefile
in drivers/devfreq don't include the entry of devfreq-event.c driver.

The original patch[1] includes the entry to build devfreq-event.c without
the build break. This build break is generated in the process of merging the
patch.
[1] https://lkml.org/lkml/2015/1/25/579


- [PATCH v10 1/7] devfreq: event: Add new devfreq_event class to provide basic
                  data for devfreq governor

CC      init/version.o
LD      init/built-in.o
drivers/built-in.o: In function `exynos_ppmu_probe':
binder.c:(.text+0x4447ec): undefined reference to `devm_devfreq_event_add_edev'
make: *** [vmlinux] Error 1

Cc: Rafael J. Wysocki <rjw@rjwysocki.net>
Cc: MyungJoo Ham <myungjoo.ham@samsung.com>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>