dt-bindings: display: amlogic, meson-vpu: Add optional power domain property

The Video Processing Unit power domain was setup by the Vendor U-Boot,
add support for an optional Power Domain phandle to setup it from the kernel.
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Tested-by: Jerome Brunet <jbrunet@baylibre.com>
Reviewed-by: Jerome Brunet <jbrunet@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1512561268-29806-2-git-send-email-narmstrong@baylibre.com

Documentation/devicetree/bindings/display/amlogic,meson-vpu.txt

@@ -64,6 +64,10 @@ Required properties:
 - reg-names: should contain the names of the previous memory regions
 - interrupts: should contain the VENC Vsync interrupt number
 
+Optional properties:
+- power-domains: Optional phandle to associated power domain as described in
+	the file ../power/power_domain.txt
+
 Required nodes:
 
 The connections to the VPU output video ports are modeled using the OF graph