dt-bindings: drm/msm/gpu: document second interconnect

Some A3xx and all A4xx Adreno GPUs do not have GMEM inside the GPU core
and must use the On Chip MEMory (OCMEM) in order to be functional.
There's a separate interconnect path that needs to be setup to OCMEM.
Let's document this second interconnect path that's available. Since
there's now two available interconnects, let's add the
interconnect-names property.

Signed-off-by: Brian Masney <masneyb@onstation.org>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>