dt-bindings: etnaviv: add slave interface clock

Newer GPU cores added a new clock input, which allows to gate the slave (AHB)
interface independently from other parts of the GPU. Add it to the supported
clocks.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/display/etnaviv/etnaviv-drm.txt

@@ -32,7 +32,9 @@ Required properties:
 - clocks: should contain one clock for entry in clock-names
   see Documentation/devicetree/bindings/clock/clock-bindings.txt
 - clock-names:
-   - "bus":    AXI/register clock
+   - "bus":    AXI/master interface clock
+   - "reg":    AHB/slave interface clock
+               (only required if GPU can gate slave interface independently)
    - "core":   GPU core clock
    - "shader": Shader clock (only required if GPU has feature PIPE_3D)