PM / devfreq: make event/exynos-ppmu DEVFREQ_EVENT_EXYNOS_PPMU tristate

The Kconfig currently controlling compilation of this code is:

config DEVFREQ_EVENT_EXYNOS_PPMU
  bool "EXYNOS PPMU (Platform Performance Monitoring Unit) DEVFREQ event Driver"

...meaning that it currently is not being built as a module by anyone.

Rather than rip out the existing modular code, Chanwoo indicated
that he'd rather see the driver offered as tristate.

I don't have the hardware for runtime validation, so this change
is only validated for compile and modpost.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Acked-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>