iio: frequency: admv4420: fix missing select REMAP_SPI in Kconfig

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>

drivers/iio/frequency/Kconfig

@@ -94,6 +94,7 @@ config ADMV1014
 config ADMV4420
 	tristate "Analog Devices ADMV4420 K Band Downconverter"
 	depends on SPI
+	select REGMAP_SPI
 	help
 	  Say yes here to build support for Analog Devices K Band
 	  Downconverter with integrated Fractional-N PLL and VCO.