iio: adc: ad7380: add support for multiple SPI lanes

iio: adc: ad7380: add support for multiple SPI lanes

Add support for multiple SPI lanes to increase throughput. The AD7380
family of ADCs have multiple SDO lines on the chip that can be used to
read each channel on a separate SPI lane. If wired up to a SPI
controller that supports it, the driver will now take advantage of this
feature. This allows reaching the maximum sample rate advertised in the
datasheet when combined with SPI offloading.

Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Reviewed-by: Marcelo Schmitt <marcelo.schmitt@analog.com>
Signed-off-by: David Lechner <dlechner@baylibre.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>