ASoC: rsrc-card: Clarify compatible value

The compatible value can be board-specific, not SoC-specific.
Add curly braces to indicate that the board type is optional.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Mark Brown <broonie@kernel.org>