This driver makes use of regmap_mmio, but does not select the required
module.
Add the missing 'select REGMAP_MMIO'.

Fixes: 4d4b3052 ("iio: dac: add support for stm32 DAC")
Signed-off-by: Javier Carrasco <javier.carrasco.cruz@gmail.com>
Link: https://patch.msgid.link/20241003-ad2s1210-select-v1-8-4019453f8c33@gmail.com
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>