samples: drivers: crypto: Align buffers

This change extends what e1e19732
(samples: drivers: crypto: Aligned AES key) has started: For certain
architectures (i.e. Si32), the alignment of the used buffers is
important as the AES HW engine uses DMA and inherits its restrictions.

Signed-off-by: Reto Schneider <reto.schneider@husqvarnagroup.com>