iio: adc: spear_adc: Use devm_platform_ioremap_resource

Avoids local boilerplate doing the same thing.
Suggested by coccinelle

CHECK   drivers/iio/adc/spear_adc.c
drivers/iio/adc/spear_adc.c:283:1-22: WARNING: Use devm_platform_ioremap_resource for st -> adc_base_spear6xx

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Alexandru Ardelean <alexandru.ardelean@analog.com>