dmaengine: pl330: improve pl330_tx_status() function

This patch adds possibility to read residue of DMA transfer. It's useful
when we want to know how many bytes have been transferred before we
terminate channel. It can take place, for example, on timeout interrupt.

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