dt-bindings: Document the dmas and dma-names properties for VC4 HDMI

These are optional, but necessary for HDMI audio support.
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
Acked-by: Rob Herring <robh@kernel.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20170227202803.12855-1-eric@anholt.net

Documentation/devicetree/bindings/display/brcm,bcm-vc4.txt

@@ -34,6 +34,9 @@ Optional properties for HDMI:
 - hpd-gpios:	The GPIO pin for HDMI hotplug detect (if it doesn't appear
 		  as an interrupt/status bit in the HDMI controller
 		  itself).  See bindings/pinctrl/brcm,bcm2835-gpio.txt
+- dmas:		Should contain one entry pointing to the DMA channel used to
+		transfer audio data
+- dma-names:	Should contain "audio-rx"
 
 Required properties for DPI:
 - compatible:	Should be "brcm,bcm2835-dpi"