dma: mmp_pdma: add support for residue reporting

A channel can accommodate more than one transaction, each consisting of
multiple descriptors, the last of which has the DCMD_ENDIRQEN bit set.

In order to report the channel's residue, we hence have to walk the
list of running descriptors, look for those which match the cookie,
and then try to find the descriptor which defines upper and lower
boundaries that embrace the current transport pointer. Once it is found,
walk forward until we find the descriptor that tells us about the end of
a transaction via a set DCMD_ENDIRQEN bit.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>