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