dt-bindings: display: msm: gmu: add optional ocmem property

Some A3xx and A4xx Adreno GPUs do not have GMEM inside the GPU core and
must use the On Chip MEMory (OCMEM) in order to be functional. Add the
optional ocmem property to the Adreno Graphics Management Unit bindings.

Signed-off-by: Brian Masney <masneyb@onstation.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>