dt-bindings: gpu: add bus clock for Mali Midgard GPUs

Some SoCs adds a bus clock gate to the Mali Midgard GPU.

Add the binding for the bus clock.
Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Signed-off-by: Clément Péron <peron.clem@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190521161102.29620-4-peron.clem@gmail.com

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

@@ -31,6 +31,12 @@ Optional properties:
 
 - clocks : Phandle to clock for the Mali Midgard device.
 
+- clock-names : Specify the names of the clocks specified in clocks
+  when multiple clocks are present.
+    * core: clock driving the GPU itself (When only one clock is present,
+      assume it's this clock.)
+    * bus: bus clock for the GPU
+
 - mali-supply : Phandle to regulator for the Mali device. Refer to
   Documentation/devicetree/bindings/regulator/regulator.txt for details.