dt-bindings: iio: dac: adi,ad56xx: Fix duplicate compatible strings

adi,ad5686.yaml and adi,ad5696.yaml duplicate all the I2C device
compatible strings with the exception of "adi,ad5337r". Since
adi,ad5686.yaml references spi-peripheral-props.yaml, drop the I2C
devices from it making it only SPI devices. Update the titles to make
the distinction clear.

Signed-off-by: Rob Herring (Arm) <robh@kernel.org>
Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://patch.msgid.link/20240910234440.1045098-1-robh@kernel.org


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