spi/spi-atmel: add dmaengine support

Add dmaengine support.

Using "has_dma_support" member of struct is used to select
the transfer mode: dmaengine or pdc.

For the dmaengine transfer mode, it supports both 8 bits and 16 bits transfer.

For the dmaengine transfer mode, if it fails to config dmaengine,
or if the message length is less than 16 bytes, it will use the PIO transfer mode.

Signed-off-by: Nicolas Ferre <[email protected]>
[[email protected]: using "has_dma_support" to select dmaengine as the spi xfer mode]
[[email protected]: fix DMA: OOPS if buffer > 4096 bytes]
[[email protected]: submit the patch]
Signed-off-by: Wenyou Yang <[email protected]>
Signed-off-by: Richard Genoud <[email protected]>
[[email protected]: update with dmaengine interface]
[[email protected]: fix __init/__devinit sections mismatch]
[[email protected]: adapt to slave_config changes]
[[email protected]: add support t0 16 bits transfer]
Tested-by: Richard Genoud <[email protected]>
Signed-off-by: Mark Brown <[email protected]>