dt-bindings:iio:dac:ad5755: txt to yaml format conversion.

Straight forward conversion.  Only fiddly bit is the XOR of
spi-cpol and spi-cpha.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Sean Nyekjaer <sean.nyekjaer@prevas.dk>
Link: https://lore.kernel.org/r/20201031184854.745828-40-jic23@kernel.org
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210424173015.534941-1-jic23@kernel.org