PM / devfreq: devbw: Switch to OPP APIs

OPP framework exposes a set of functions that could be used to fetch
the list of supported bandwidth values from DT and to query the list
for an appropriate value while honoring requests from
governors/clients.
Use OPP table APIs and get rid of frequency table logic to make the
code more concise and upstream friendly.

Change-Id: I120ae7230bdbfaaa9523263c65a8b84470583071
Signed-off-by: Rohit Gupta <rohgup@codeaurora.org>