dt-bindings: iio: adc: add TI LMP92064 controller

Add binding documentation for the TI LMP92064 dual channel SPI ADC.

Signed-off-by: Leonard Göhrs <l.goehrs@pengutronix.de>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20221128133503.1355898-1-l.goehrs@pengutronix.de
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>