spi/davinci: Support DMA transfers larger than 65535 words

The current davinci SPI driver, in DMA mode, is limited to 65535
words for a single transfer.  Modify the driver by configuring a
3 dimensional EDMA transfer to support up to 65535x65535
words.

Signed-off-by: Michael Williamson <michael.williamson@criticallink.com>
Tested-by: Stefano Babic <sbabic@denx.de>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>