iio: dac: ltc1660: add missing select REGMAP_SPI in Kconfig

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

Fixes: 8316ceb ("iio: dac: add support for ltc1660")
Signed-off-by: Javier Carrasco <[email protected]>
Link: https://patch.msgid.link/[email protected]
Cc: <[email protected]>
Signed-off-by: Jonathan Cameron <[email protected]>

Author: javiercarrascocruz

drivers/iio/dac/Kconfig

@@ -358,6 +358,7 @@ config LPC18XX_DAC
 config LTC1660
 	tristate "Linear Technology LTC1660/LTC1665 DAC SPI driver"
 	depends on SPI
+	select REGMAP_SPI
 	help
 	  Say yes here to build support for Linear Technology
 	  LTC1660 and LTC1665 Digital to Analog Converters.