media: dt-bindings: qcom,venus: document firmware-name

Venus Linux driver loads firmware based on firmware-name property and
some DTS already have it:

  msm8996-oneplus3.dtb: video-codec@c00000: Unevaluated properties are not allowed ('firmware-name', 'interconnect-names', 'interconnects' were unexpected)

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>