iio: resolver: ad2s1210 add missing select REGMAP in Kconfig

iio: resolver: ad2s1210 add missing select REGMAP in Kconfig

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

Fixes: b3689e14415a ("staging: iio: resolver: ad2s1210: use regmap for config registers")
Signed-off-by: Javier Carrasco <javier.carrasco.cruz@gmail.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20241003-ad2s1210-select-v1-1-4019453f8c33@gmail.com
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>