PM / devfreq: make exynos-bus ARM_EXYNOS_BUS_DEVFREQ tristate

The Kconfig currently controlling compilation of this code is:

devfreq/Kconfig:config ARM_EXYNOS_BUS_DEVFREQ
devfreq/Kconfig:        bool "ARM EXYNOS Generic Memory Bus DEVFREQ 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>