spi: at91-usart: add DMA support

This patch adds support for DMA. Transfers are done with dma only if
they are longer than 16 bytes in order to achieve a better performance.
DMA setup introduces a little overhead and for transfers shorter than 16
bytes there is no performance improvement.

Signed-off-by: Radu Pirea <radu_nicolae.pirea@upb.ro>
Signed-off-by: Mark Brown <broonie@kernel.org>