dmaengine/ep93xx_dma: Implement double buffering for M2M DMA channels

Add double buffering support for M2M DMA channels. Implement this by using EP93xx
M2M DMA Buffer and Control Finite State Machines to be sure that we are not
disabling the channel when it's actually operating.

Signed-off-by: Rafal Prylowski <prylowski@metasoft.pl>
Tested-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Acked-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Acked-by: Mika Westerberg <mika.westerberg@iki.fi>
Signed-off-by: Ryan Mallon <rmallon@gmail.com>