iio: adc: at91-sama5d2: Fix buffer alignment in iio_push_to_buffers_with_timestamp()

To make code more readable, use a structure to express the channel
layout and ensure the timestamp is 8 byte aligned.

Found during an audit of all calls of this function.

Fixes: 5e1a1da0 ("iio: adc: at91-sama5d2_adc: add hw trigger and buffer support")
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Eugen Hristev <eugen.hristev@microchip.com>
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20210613152301.571002-2-jic23@kernel.org

drivers/iio/adc/at91-sama5d2_adc.c

@@ -403,7 +403,8 @@ struct at91_adc_state {
 	struct at91_adc_dma		dma_st;
 	struct at91_adc_touch		touch_st;
 	struct iio_dev			*indio_dev;
-	u16				buffer[AT91_BUFFER_MAX_HWORDS];
+	/* Ensure naturally aligned timestamp */
+	u16				buffer[AT91_BUFFER_MAX_HWORDS] __aligned(8);
 	/*
 	 * lock to prevent concurrent 'single conversion' requests through
 	 * sysfs.