The spi controller nodes omit the dma controller/channel references, add
it.

This does not yet enable DMA for SPI transfers, as the spi-sun6i driver
lacks support for DMA, but always uses PIO to the FIFO.
Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>