ASoC: dt-bindings: renesas,rsnd: tidyup rcar_sound,src

ASoC: dt-bindings: renesas,rsnd: tidyup rcar_sound,src

Some SRC is not implemented on some SoC, thus
interrupts/dmas/dma-names are not mandatory.
This patch solve it. Without this patch we will get below error
when 'make DT_CHECKER_FLAGS=-m dt_binding_check'.

dtschema/dtc warnings/errors:
${LINUX}/Documentation/devicetree/bindings/sound/renesas,rsnd.example.dtb: \
 sound@ec500000: Unevaluated properties are not allowed ('rcar_sound,src' was unexpected)
	From schema: /builds/robherring/dt-review-ci/linux/Documentation/devicetree/bindings/sound/renesas,rsnd.yaml

Link: https://lore.kernel.org/r/167344317928.394453.14105689826645262807.robh@kernel.org
Reported-by: Rob Herring <robh@kernel.org>
Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Link: https://lore.kernel.org/r/87fsc17rfa.wl-kuninori.morimoto.gx@renesas.com
Signed-off-by: Mark Brown <broonie@kernel.org>