Add binding Doc for this SPI DAC.

The driver was perviously posted but was missing the DT binding document.
https://lore.kernel.org/all/20221106165928.223318-1-angelo.dureghello@timesys.com/Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Angelo Dureghello <angelo.dureghello@timesys.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230115173958.134201-1-jic23@kernel.org