iio: adc: ad_sigma_delta: Select IIO_BUFFER_DMAENGINE and SPI_OFFLOAD

iio: adc: ad_sigma_delta: Select IIO_BUFFER_DMAENGINE and SPI_OFFLOAD

CONFIG_AD_SIGMA_DELTA uses several symbols that it does not explicitly
select. If no other enabled driver selects them, the build fails with
either a linker failure if the driver is built in or a modpost failure
if the driver is a module.

  ld.lld: error: undefined symbol: devm_spi_offload_rx_stream_request_dma_chan
  ld.lld: error: undefined symbol: devm_iio_dmaengine_buffer_setup_with_handle
  ld.lld: error: undefined symbol: devm_spi_offload_trigger_get
  ld.lld: error: undefined symbol: devm_spi_offload_get
  ld.lld: error: undefined symbol: spi_offload_trigger_enable
  ld.lld: error: undefined symbol: spi_offload_trigger_disable

Select the necessary Kconfig symbols to include these functions in the
build to clear up the errors.

Fixes: 219da3ea842a ("iio: adc: ad_sigma_delta: add SPI offload support")
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20250714-iio-ad_sigma_delta-fix-kconfig-selects-v1-1-32e0d6da0423@kernel.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>