iio: adc: xilinx-xadc: use devm_platform_ioremap_resource

Reduces local boilerplate.
Suggested by coccinelle.
 CHECK   drivers/iio/adc/xilinx-xadc-core.c
drivers/iio/adc/xilinx-xadc-core.c:1184:1-11: WARNING: Use devm_platform_ioremap_resource for xadc -> base

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Manish Narani <manish.narani@xilinx.com>
Reviewed-by: Sven Van Asbroeck <TheSven73@gmail.com>