iio: adc: ad7606: explicit timestamp alignment

Use struct with aligned_s64 timestamp to make timestamp alignment
explicit. Technically, what we have works because for all known
architectures, IIO_DMA_MINALIGN is a multiple of __alignof__(s64).
But this way, we don't have to make people read the comments to know
why there are extra elements in each buffer.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Link: https://patch.msgid.link/20250428-iio-adc-ad7606-fix-buffer-alignment-v1-1-88dfc57e5df0@baylibre.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>