ASoC: rsnd: tidyup for SSIU subnode

commit da48a6eb82 ("ASoC: rsnd: add SSIU BUSIF support for
Document") updated Documentation for SSIU, but 1) we want to
keep old/deprecated DMA description, 2) it is missing SSIU
subnode properties. This patch tidyup these

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/renesas,rsnd.txt

@@ -327,8 +327,13 @@ SSI subnode properties:
 - no-busif			: BUSIF is not ussed when [mem -> SSI] via DMA case
 - dma				: Should contain Audio DMAC entry
 - dma-names			: SSI  case "rx"  (=playback), "tx"  (=capture)
+				  Deprecated: see SSIU subnode properties
 				  SSIU case "rxu" (=playback), "txu" (=capture)
 
+SSIU subnode properties:
+- dma				: Should contain Audio DMAC entry
+- dma-names			: "rx" (=playback), "tx" (=capture)
+
 SRC subnode properties:
 - dma				: Should contain Audio DMAC entry
 - dma-names			: "rx" (=playback), "tx" (=capture)