dt-bindings: gpu: mali-utgard: add hisilicon,hi6220-mali compatible

The Hisilicon hi6220 uses a Mali-450MP4 with 4 PPs, so add
a compatible for it.

Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Philipp Zabel <p.zabel@pengutronix.de>
Cc: dri-devel@lists.freedesktop.org
Cc: devicetree@vger.kernel.org
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Peter Griffin <peter.griffin@linaro.org>
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Rob Herring <robh@kernel.org>

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

@@ -24,6 +24,7 @@ Required properties:
       + rockchip,rk3228-mali
       + rockchip,rk3328-mali
       + stericsson,db8500-mali
+      + hisilicon,hi6220-mali
 
   - reg: Physical base address and length of the GPU registers
 
@@ -98,6 +99,10 @@ to specify one more vendor-specific compatible, among:
       * interrupt-names and interrupts:
         + combined: combined interrupt of all of the above lines
 
+  - hisilicon,hi6220-mali
+    Required properties:
+      * resets: phandles to the reset lines for the GPU
+
 Example:
 
 mali: gpu@1c40000 {