msm: kgsl: Read DDR specific strings for the bus frequency values

Check DDR specific properties for qcom,bus-freq, qcom,bus-min and
qcom,bux-max for each power level and use those if they exist for
the current DDR type otherwise fall back to the default.

Change-Id: Ic0dedbad1a09d5fc5aed7310448acb6519d44bc9
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>