iio: dac: lpc18xx: Use devm_platform_ioremap_resource

Reduce boilerplate.
Suggested by coccinelle
  CHECK   drivers/iio/dac/lpc18xx_dac.c
drivers/iio/dac/lpc18xx_dac.c:121:1-10: WARNING: Use devm_platform_ioremap_resource for dac -> base

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
CC: Joachim Eastwood <manabian@gmail.com>