dt-bindings: iio: adc: ad7606: Remove spi-cpha from required

The documentation is erroneously stating that spi-cpha is required, and
the example is erroneously setting both spi-cpol and spi-cpha. According
to the datasheet, only cpol should be set.

On zedboard for instance, setting the devicetree as in the example will
simply not work.

Fixes: 416f882c ("dt-bindings: iio: adc: Migrate AD7606 documentation to yaml")
Fixes: 6e33a125 ("dt-bindings: iio: adc: Add docs for AD7606 ADC")
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Guillaume Stols <gstols@baylibre.com>
Link: https://patch.msgid.link/20241015-ad7606_add_iio_backend_support-v5-1-654faf1ae08c@baylibre.com


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