msm: sde: rot: add parsing logic to get reg bw vote from DT

Add parsing code to get the rotator register bw vote settings
from the device tree, so that it can be extended to other targets.

Change-Id: Icb64467a1380c614745dc57a16287c889d33023a
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>