dt-bindings: gpu: mali: Add optional memory-region

The reserved memory bindings allow us to specify which memory areas our
buffers can be allocated from.

Let's use it.
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/gpu/arm,mali-utgard.txt

@@ -35,6 +35,10 @@ Optional properties:
   - interrupt-names and interrupts:
     * pmu: Power Management Unit interrupt, if implemented in hardware
 
+  - memory-region:
+    Memory region to allocate from, as defined in
+    Documentation/devicetree/bindi/reserved-memory/reserved-memory.txt
+
 Vendor-specific bindings
 ------------------------