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>

Documentation/devicetree/bindings/sound/renesas,rsrc-card.txt

@@ -4,8 +4,8 @@ Renesas Sampling Rate Convert Sound Card specifies audio DAI connections of SoC
 
 Required properties:
 
-- compatible				: "renesas,rsrc-card,<board>"
-					  Examples with soctypes are:
+- compatible				: "renesas,rsrc-card{,<board>}"
+					  Examples with boards are:
 					    - "renesas,rsrc-card"
 					    - "renesas,rsrc-card,lager"
 					    - "renesas,rsrc-card,koelsch"