iio: mcp9600: Add support for thermocouple-type

iio: mcp9600: Add support for thermocouple-type

dt-bindings documentation for this driver claims to support
thermocouple-type, but the driver does not actually make use of
the property.

Implement usage of the property to configure the chip for the
selected thermocouple-type.

Signed-off-by: Ben Collins <bcollins@watter.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20250822-upstream-changes-v8-5-40bb1739e3e2@watter.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>