ASoC: rcar: tidyup simple-card example for CPU node

commit a5702e1c ("ASoC: rsnd: Drop unit-addresses without reg
properties") modifies simple-card multi CPU nodes.
But, naming of "cpu-x" breaks probing.
Let's add reg = <x>; instead of renaming node.

Reported-by: Hiroyuki Yokoyama <hiroyuki.yokoyama.vx@renesas.com>
CC: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Mark Brown <broonie@kernel.org>