dt-bindings: devfreq: Add DT bindings to support OPP BW values

This patch adds DT bindings for the list of supported bandwidth
values added in device tree.

Change-Id: I0e8fd3ba3aeb6c98bc027a621455288606683fd3
Signed-off-by: Rama Aparna Mallavarapu <aparnam@codeaurora.org>