ARM: sun7i: dt: enable DMA on SPI

All of our SPI controllers support DMA transfers, so let's add the
properties here so they can be used when it's best to do so.

Signed-off-by: Emilio López <emilio@elopez.com.ar>
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>