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

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.com
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>