spi: SAM support SPI transfers with DMA

For larger transfers DMA can be used enabling other tasks
to continue running. A threshold of 32 byte transfers
is about right and is defined threshold value for using DMA.

This does not currently support multiple SPI transactions changing
chip select with DMA (though the hardware supports this) currently.
Instead opting for the simpler first change of enabling one shot
DMA SPI transfers for those where the size warrants it.

Adds the loopback binding option to enable the spi_loopback test.

Signed-off-by: Tom Burdick <thomas.burdick@intel.com>