iio:dac:ad5064: Add support for the ad5629r and ad5669r

The ad5629r and ad5669r are the I2C variants of the ad5628 and ad5668. Since the
ad5064 driver currently only supports SPI based devices the major part of this
patch focuses on adding support for I2C based devices. Adding support for the
actual parts boils down to adding entries for them to the device id table.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>