ARM: dts: renesas: r8a7778: Add missing reg-names to sound node

Sound Driver requires "reg-names" to get register info.  Current driver
tries to get register info via "reg" instead of "reg-names" as backup
plan, but this support will be removed soon.
Use "reg-namess" for r8a7778 sound.
Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/87cyszpwmp.wl-kuninori.morimoto.gx@renesas.comSigned-off-by: Geert Uytterhoeven <geert+renesas@glider.be>

arch/arm/boot/dts/renesas/r8a7778.dtsi

@@ -255,6 +255,8 @@ rcar_sound: sound@ffd90000 {
 		reg =	<0xffd90000 0x1000>,	/* SRU */
 			<0xffd91000 0x240>,	/* SSI */
 			<0xfffe0000 0x24>;	/* ADG */
+		reg-names = "sru", "ssi", "adg";
+
 		clocks = <&mstp3_clks R8A7778_CLK_SSI8>,
 			<&mstp3_clks R8A7778_CLK_SSI7>,
 			<&mstp3_clks R8A7778_CLK_SSI6>,