iio: dac: ti-dac7612: Fix alignment for DMA safety

____cacheline_aligned is an insufficient guarantee for non-coherent DMA
on platforms with 128 byte cachelines above L1.  Switch to the updated
IIO_DMA_MINALIGN definition.

Updated help text to 'may' require buffers to be in their own cacheline.

Fixes: 977724d2 ("iio:dac:ti-dac7612: Add driver for Texas Instruments DAC7612")
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Ricardo Ribalda <ribalda@kernel.org>
Acked-by: Nuno Sá <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20220508175712.647246-65-jic23@kernel.org