spi/ep93xx: add DMA support

spi/ep93xx: add DMA support

This patch adds DMA support for the EP93xx SPI driver. By default the DMA is
not enabled but it can be enabled by setting ep93xx_spi_info.use_dma to true
in board configuration file.

Note that the SPI driver still uses PIO for small transfers (<= 8 bytes) for
performance reasons.

Signed-off-by: Mika Westerberg <[email protected]>
Acked-by: H Hartley Sweeten <[email protected]>
Cc: Grant Likely <[email protected]>
Acked-by: Grant Likely <[email protected]>
Signed-off-by: Vinod Koul <[email protected]>