Merged in extent3d/nuttx/dma (pull request #597)

SAMDL DMA fixes and experimental SPI support

* SAMDL: Fix DMA controller support

* SAMDL: Added experimental DMA support to SPI driver. spi_exchange() uses a pair of DMA channels for TX and RX

Approved-by: Gregory Nutt <gnutt@nuttx.org>