iio:dac:lpc18xx_dac: Swap from of* to mod_devicetable.h

This driver never used anything from the of specific headers.
mod_devicetable.h provides the struct of_device_id definition.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>