ASoC: gtm601: Document GTM601 bindings

Add small documentation for GTM601 UMTS modem audio interface.

Signed-off-by: Marek Belisko <marek@goldelico.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

diff --git a/Documentation/devicetree/bindings/sound/gtm601.txt b/Documentation/devicetree/bindings/sound/gtm601.txt
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+GTM601 UMTS modem audio interface CODEC
+
+This device has no configuration interface. Sample rate is fixed - 8kHz.
+
+Required properties:
+
+  - compatible : "option,gtm601"
+
+Example:
+
+codec: gtm601_codec {
+       compatible = "option,gtm601";
+};