spi/rockchip: Mark DMA as optional

The Rockchip SPI controller works fine without DMA (aside from a few
warnings).  The DMA property even implies this, saying:

  DMA request names should include "tx" and "rx" if present.

Officially mark the properties as optional.

Signed-off-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Mark Brown <broonie@kernel.org>