iio: adc: lpc18xx: use devm_platform_ioremap_resource

Avoid local boilerplate.
Identified by coccinelle

CHECK   drivers/iio/adc/lpc18xx_adc.c
drivers/iio/adc/lpc18xx_adc.c:137:1-10: WARNING: Use devm_platform_ioremap_resource for adc -> base

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