iio/dac: convert ltc2632.txt to lltc,ltc2632.yaml

Conversion of the ltc2632 to yaml format and name the file to
'lltc,ltc2632.yaml'.

Signed-off-by: Chris Ruehl <chris.ruehl@gtsys.com.hk>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>