dmaengine: sa11x0-dma: add cyclic DMA support

Add support for cyclic DMA on sa11x0 platforms.  This follows the
discussed behaviour that the callback will be called at some point
after period expires, and may coalesce multiple period expiries into
one callback (due to the tasklet behaviour.)

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>