dt-bindings: iio: dac: ad3552r: fix maximum spi speed

Fix maximum SPI clock speed, as per datasheet (Rev. B, page 6).

Fixes: b0a96c5f ("dt-bindings: iio: dac: Add adi,ad3552r.yaml")
Cc: stable@vger.kernel.org
Signed-off-by: Angelo Dureghello <adureghello@baylibre.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://patch.msgid.link/20241003-wip-bl-ad3552r-axi-v0-iio-testing-v4-4-ceb157487329@baylibre.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>