iio: dac: ad5758: declare missing of table

Add missing <of_device_id> table for SPI driver relying on SPI
device match since compatible is in a DT binding or in a DTS.

Before this patch:
modinfo drivers/iio/dac/ad5758.ko | grep alias
alias:          spi:ad5758

After this patch:
modinfo drivers/iio/dac/ad5758.ko | grep alias
alias:          spi:ad5758
alias:          of:N*T*Cadi,ad5758C*
alias:          of:N*T*Cadi,ad5758

Reported-by: Javier Martinez Canillas <javier@dowhile0.org>
Signed-off-by: Daniel Gomez <dagmcr@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>