dt-bindings: drm/msm/a6xx: Document interconnect properties for GPU

Add documentation for the interconnect and interconnect-names bindings
for the GPU node as detailed by bindings/interconnect/interconnect.txt.

Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Georgi Djakov <georgi.djakov@linaro.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>