dma: Indicate residue granularity in dma_slave_caps

dma: Indicate residue granularity in dma_slave_caps

This patch adds a new field to the dma_slave_caps struct which indicates the
granularity with which the driver is able to update the residue field of the
dma_tx_state struct. Making this information available to dmaengine users allows
them to make better decisions on how to operate. E.g. for audio certain features
like wakeup less operation or timer based scheduling only make sense and work
correctly if the reported residue is fine-grained enough.

Right now four different levels of granularity are supported:
	* DESCRIPTOR: The DMA channel is only able to tell whether a descriptor has
	  been completed or not, which means residue reporting is not supported by
	  this channel. The residue field of the dma_tx_state field will always be
	  0.
	* SEGMENT: The DMA channel updates the residue field after each successfully
	  completed segment of the transfer (For cyclic transfers this is after each
	  period). This is typically implemented by having the hardware generate an
	  interrupt after each transferred segment and then the drivers updates the
	  outstanding residue by the size of the segment. Another possibility is if
	  the hardware supports SG and the segment descriptor has a field which gets
	  set after the segment has been completed. The driver then counts the
	  number of segments without the flag set to compute the residue.
	* BURST: The DMA channel updates the residue field after each transferred
	  burst. This is typically only supported if the hardware has a progress
	  register of some sort (E.g. a register with the current read/write address
	  or a register with the amount of bursts/beats/bytes that have been
	  transferred or still need to be transferred).

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Mark Brown <broonie@linaro.org>