PM / devfreq: Make the CPUBW voting device driver to be more generic

Making the driver more generic would allow it to be reused for voting for
BW requirements of other devices like GPU, etc.

Instead of cpubw and qcom,cpubw, the driver and the device are now called
devbw and qcom,devbw respectively.

Change-Id: I49e5a5ba6d46517da19d013413abd471a730af29
Signed-off-by: Saravana Kannan <skannan@codeaurora.org>