msm: devfreq_cpubw: Move devfreq_cpubw to drivers/devfreq

In order to share devfreq_cpubw among all targets, it needs to be
moved outside arch/arm/mach-msm directory. Move the file and
Kconfig to drivers/devfreq.

Change-Id: I4dd476b315b04f84b07855bec3ed5b6061549ea9
Signed-off-by: Junjie Wu <junjiew@codeaurora.org>