dt-bindings: iio/temperature: Add thermocouple types (and doc)

This patch introduces common thermocouple types used by various
temperature sensors. Also a brief documentation explaining this
"thermocouple-type" property.
Signed-off-by: Patrick Havelange <patrick.havelange@essensium.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/devicetree/bindings/iio/temperature/temperature-bindings.txt

+If the temperature sensor device can be configured to use some specific
+thermocouple type, you can use the defined types provided in the file
+"include/dt-bindings/iio/temperature/thermocouple.h".
+
+Property:
+thermocouple-type:	A single cell representing the type of the thermocouple
+			used by the device.

include/dt-bindings/iio/temperature/thermocouple.h

+/* SPDX-License-Identifier: GPL-2.0 */
+
+#ifndef _DT_BINDINGS_TEMPERATURE_THERMOCOUPLE_H
+#define _DT_BINDINGS_TEMPERATURE_THERMOCOUPLE_H
+
+
+#define THERMOCOUPLE_TYPE_B	0x00
+#define THERMOCOUPLE_TYPE_E	0x01
+#define THERMOCOUPLE_TYPE_J	0x02
+#define THERMOCOUPLE_TYPE_K	0x03
+#define THERMOCOUPLE_TYPE_N	0x04
+#define THERMOCOUPLE_TYPE_R	0x05
+#define THERMOCOUPLE_TYPE_S	0x06
+#define THERMOCOUPLE_TYPE_T	0x07
+
+#endif /* _DT_BINDINGS_TEMPERATURE_THERMOCOUPLE_H */