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

Fixes: b59c0415 ("iio: frequency: admv4420.c: Add support for ADMV4420")
Signed-off-by: Javier Carrasco <javier.carrasco.cruz@gmail.com>
Link: https://patch.msgid.link/20241007-ad2s1210-select-v2-2-7345d228040f@gmail.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>