ASoC: rsnd: add missing reg/reg-name at Document

reg is missing "Audio DMAC peri peri" part,
and reg-name itself is missing at Document.
This patch add these.
Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Mark Brown <broonie@kernel.org>

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

@@ -282,7 +282,10 @@ Required properties:
 - reg				: Should contain the register physical address.
 				  required register is
 				   SRU/ADG/SSI      if generation1
-				   SRU/ADG/SSIU/SSI if generation2
+				   SRU/ADG/SSIU/SSI/AUDIO-DMAC-periperi if generation2/generation3
+- reg-names			: Should contain the register names.
+				   scu/adg/ssi	if generation1
+				   scu/adg/ssiu/ssi/audmapp if generation2/generation3
 - rcar_sound,ssi		: Should contain SSI feature.
 				  The number of SSI subnode should be same as HW.
 				  see below for detail.