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>

Documentation/devicetree/bindings/sound/gtm601.txt

+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";
+};