iio: dac: ad5770r: add missing select REGMAP_SPI in Kconfig

iio: dac: ad5770r: add missing select REGMAP_SPI in Kconfig

commit bcdab6f74c91cda19714354fd4e9e3ef3c9a78b3 upstream.

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

Fixes: cbbb819837f6 ("iio: dac: ad5770r: Add AD5770R support")
Signed-off-by: Javier Carrasco <javier.carrasco.cruz@gmail.com>
Link: https://patch.msgid.link/20241003-ad2s1210-select-v1-6-4019453f8c33@gmail.com
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>