dt-bindings: trivial-devices: document the Sierra Wireless mangOH Green SPI IoT interface

Document the Sierra Wireless mangOH Green SPI IoT interface as a trivial
device.

This fixes the following check:
qcom-mdm9615-wp8548-mangoh-green.dtb: /soc/gsbi@16200000/spi@16280000/spi@0: failed to match any schema with compatible: ['swir,mangoh-iotport-spi']

Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20240710-topic-mdm9615-mangoh-iotport-spi-bindings-v1-1-3efe20cfea8a@linaro.org


Signed-off-by: Rob Herring (Arm) <robh@kernel.org>