dmaengine: pl330: add DMA_PAUSE feature

DMA_PAUSE command is used for halting DMA transfer on chosen channel.
It can be useful when we want to safely read residue before terminating
all requests on channel. Otherwise there can be situation when some data
is transferred before channel termination but after reading residue,
which obviously results with data loss. To avoid this situation we can
pause channel, read residue and then terminate all requests.
This scenario is common, for example, in serial port drivers.

Signed-off-by: Robert Baldyga <r.baldyga@samsung.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>