ARM: dts: msm: Connect KGSL and gpubw device for MSM8939

qcom,gpubw device represents KGSL bus bandwidth votes. KGSL driver
needs to have an access to the corresponding device structure.

Change-Id: I206e3a269023f69a0a51766b8d61c3b838725602
Signed-off-by: Mihir Patel <mihirp@codeaurora.org>